So, there are a few reasons as to why I have ended Boss Ideas 5 and started on #7, despite there still not being a theme. Some examples of why I have can be seen in a short list below:

1. Layout changes:
-The most boring of the lot to be honest. I found that tactics for fighting these currently-nonexistent bosses were merging to the useless-but-kinda-fun facts, so decided to divide those sections. The GelTech, due to this not being directly connected to Boss Ideas 4 & 5, doesn't exist for these bosses so they may not have any interesting lore to talk about. Finally, debuffs were confusing and everywhere so I'm adding a 'Debuff Almanac' to the beginning of every page to log all of the debuffs used, also to avoid clutter on individual boss' pages and for me to track which ones I used in case I want to use them again in the future.

2. Animated Sprites
-Yes, there are indeed now some animated sprites among these. However, the only catch is that some of the larger sprites and some worms may not be animated either due complexity or their movements being because of segments (and thus the only animations would be anything the individual segments do alone). Though, despite these, there are now a lot of animated sprites; so no more white borders and still frames in most cases :)

3. Lore
-Since lore is still important to an extent, there's still some lore stuff to talk about. Firstly, this doesn't take place at the same time as Boss Ideas 4 & 5 (Which I'll dub Project Chaos for an easier name) and instead takes place a long time after; with the King returning and the kingdom having relocated itself many hundreds of miles away for some 'unknown reason'. This means that, while lore may intertwine with that of Chaos', a lot of that project's important bosses probably won't be making a return: either having been slain or leaving with the kingdom. (Done mostly to avoid re-using older bosses over and over again :P)

4. Changes To Post-Moon Lord/ Chaos Mode
-The final major change. While Chaos Mode still exists it doesn't have the same problems as before with the rather lacking damage output. Damage has been fixed from here on out and thus bosses will no longer ignore 50% of player defences and no longer deal 1.5x damage; instead dealing what their stats would suggest. They've also been balanced closer to Calamity standards because, if this ever became an actual mod, compatibility is important :P

Any of the debuffs seen in this page or ones potentially brought over from previous pages. Some have been changed a little if brought over, however.

Chaotic Ember
Tooltip - "Unstable energies consume you"
Effect - Deals 50 damage per second
Extra - All Chaos Mode enemies inflict this debuff. Usually lasts no longer than a second.

Crimson Embers
Tooltip - "Bright red flames burn around you"
Effect - Deals 15 damage per second

Heavy Gravity
Tooltip - "Gravity feels stronger"
Effect - Flight time reduced by 30% and flight speed and acceleration reduced by 50%
Extra - Often used for larger bosses. Most cases have it inflicted at the start of a fight and lasting until the boss in question leaves

Hypersensitivity
Tooltip - "Your senses have been heightened drastically"
Effect - Weakened hunter potion effect, though also reduces player's attack by 5% and being hit inflicts the Stunned debuff
Extra - Usually caused by enemies that inflict Electrified or a version of Venom.

Ichorian Spark
Tooltip - "The Ichor is burning..."
Effect - Deals 8 damage per second and reduces the player's defence by 30

Lethal Venom
Tooltip - "Better find that antidote"
Effect - Deals 6 damage per second for 10 seconds, then increases by 1 damage per second every 8 seconds. Lasts until it is either cured or the player dies.
Extra - Can be cured by the Nurse or the Antivenom Vial

Lovestruck
Tooltip - "You've been blinded by love"
Effect - The player cannot attack, but can still use non-damaging items. Movement speed also reduced by 15%

Nature's Fury
Tooltip - "The power of nature directed at you"
Effect - Reduces player defence by 20 and deals 10 damage per second.

Stunned
Tooltip - "You have frozen up..."
Effect - Player cannot use items, loses 10 defence, and movement, jump and flight speed reduced by 60%
Extra - Doesn't usually last long and often caused by being hit with the Hypersensitivity debuff

Unstable Energy
Tooltip - "The energies absorbed are too strong to handle"
Effect - Deals 20 damage per second, reduces player's defence by 20, and removes any additional damage reduction effects outside of the player's defence stat.

A biome filled with dangerous monsters, surrounded by a near-unbreakable shell. It may seem quiet at first glance, but some of the most dangerous entities exist within its borders.

Appearance: A large, vaguely semi-circular biome made of dark-grey stone. Numerous pillars hold up the ceiling and, in the very centre of the room, there is a small pedestal with a strange fountain in its centre. The pillars and pedestal all seem man-made, with details and decorations, but the rest of the depths appear rugged and untouched; with countless stalactites and stalagmites protruding from the floor and ceiling.

Type: Large mini-biome
Spawn: World generation, centre of world, cavern layer.
Special: 'Creative Shock' debuff applied when near the biome. Blocks cannot be destroyed without first beating one of the bosses seen far later in the game.

Bosses:
Pre-Hardmode:
Molten Slime* - Large rocky slime with magma visible from within the cracks in its body.
Terminus - The Cragged Depths' main occupant. Can be challenged upon defeating Skeletron, but only until defeated.
Hardmode:
Post-Moon Lord:
Void Terminus - Terminus at peak power. Otherworldly energies flow through him, granting him abilities far beyond his own capabilities.
Chaos Mode:
Phantasm - A ghostly monolith of otherworldly energy, brought to life

[*Can be summoned elsewhere as well]

Enemies: [None, Except Bosses]

Items: [Boss Drops Only]

Extra Info:
-The biome's blocks, despite already having a barrier on them until defeating Void Terminus, also have a pickaxe resistance of 2,000; higher than any vanilla block and almost 10 times higher than any vanilla pickaxe can mine.
-Despite being a mini-biome there are no enemies or items exclusive to it, only bosses and their own respective drops.
-Even though numerous bosses are summoned in this biome, none of them actually need to remain confined to it whilst fighting them. Therefore, a good suggestion would be to take any bosses out of the biome and make a tunnel to the surface or to a larger arena nearby to grant the player more room to fight them.
--This is especially true for bosses like Phantasmaw and Void Terminus who have things like large hitboxes, fast movements, or lots of projectile-based attacks.
[Added: 5/8/19]

Appearance: A brown, rugged-looking slime coated in large spikes. A red substance, likely some sort of magma, can be seen flowing between some of the cracks in its body.

Summon: Use the Molten Stone in the Cragged Depths, Underworld, or Cavern Layer. While the boss can be taken anywhere after being summoned, its limited mobility makes taking it to different biomes unnecessarily complicated. The Molten Stone can only be crafted post-EoW/BoC because it uses either Shadow Scales or Tissue Samples as materials.

Fought: Pre-Hardmode
Specification: Post-Eater of Worlds/Brain of Cthulhu
Special?: No
Progression: No

Stats:
Health: 4,000 (6,000)
Attack:
-55 (80) (Melee) [Debuff: On Fire! (100%)]
-45 (67) (Fireball, Rocks)
Defence: 14

Attacks:
Jumps towards the player in a fashion similar to King Slime, though unlike King Slime it may sometimes ignore blocks if it needs to reach the player. Its pattern has it jump 3 times normally before performing a larger jump. At the end of the larger jump it performs 1 of 2 actions:
-Summons numerous rocks which fall slowly from the ceiling above the player
-May release 2 bouncing fireballs; 1 to the left and 1 to the right. These remain active until they've bounced up to 3 times or get too far from the player.
As the Molten Slime takes damage it will do 2 things. Firstly, it may release 2 Lava Slimes which act as they normally would (but don't turn into lava upon their destruction). Secondly, the Molten Slime speeds up the lower its health gets, both increasing its attack frequency and movement speed.

Defeat: The Molten Slime bursts into gibs of its rocky shell, and also releases numerous red particles which slowly fall downwards before disappearing. Any Lava Slimes still active remain the same as in the fight.

Drops: A flail, a magic staff that drops rocks on foes, and a summoner staff that summons mini Molten Slimes that act like a stronger Slime Staff. In Expert Mode it also drops an accessory that provides 3 defence, immunity to fire blocks, and a dash attack that can ignite enemies.

Hints:
-As it lacks a teleportation ability, the player could make a platform roughly 40 blocks from the ground and instantly become immune to all of the Molten Slime's abilities outside of the falling rock attack.
--The falling rock attack is the most problematic from a 'cheese' standpoint because it also stops the player simply making a rope tower and shooting at the slime below.
-Lightning boots allow the player to outrun the Molten Slime, even at low health, making them great for distancing yourself from the boss

Extra Info:
-The Molten Slime was the first boss to actually be animated and was originally planned to be a normal enemy before being moved to a mini-boss and then, finally, to a mid Pre-Hardmode boss.
--This is because, despite appearing on the previous Boss Ideas, both Callie and Arterion Prime were animated far later and are in fact predated by bosses 2, 3, 4 and 5 as well.
[Added: 5/8/19]

Appearance: Appears to be a blue-haired human, though a few things suggest otherwise. She wears a mostly-plain white and black outfit, with the only notable accessory being a shiny blue medallion that matches the colour of her hair and eyes. Both her eyes and the medallion seem to glow occasionally.

Summon: Use the Shimmering Dew during the day. The boss doesn't actually despawn if it reaches nighttime, meaning that the boss can be summoned without consequence at any time of the day without the worry of it despawning or enraging at night. The Shimmering Dew requires few items to craft, though a fair amount of Souls of Light are needed.

Fought: Hardmode
Specification: Pre-Mechanicals
Special?: No
Progression?: Yes, Later Bosses

Stats:
Health:
-14,000 (22,000) (Lightseeker)
-200 (350) (Illuminant Vision)
Attack:
-50 (80) (Lightseeker Melee, Illuminant Vision Melee)
-35 (60) (Light Shards, Ancient Light)
Defence:
-10 (Lightseeker)
-0 (Illuminant Vision)

Attacks:
The Lightseeker herself will usually get relatively close to the player before attacking, usually by just running and jumping as opposed to any sort of flight or teleportation methods like other bosses may use. Her movement speed is based on the distance between her and the player; with longer distances having her run faster to reduce the time needed to close the gap. While attacking she will jump up and hang in the air whilst attacking, falling down again once the attack has finished.
-She may launch an arc of 5 'light shards' at the player. These move at roughly the same speed as Spazmatism's fireballs, but the projectiles themselves are actually smaller. They do not pass through blocks.
-She may open 3 portals close to the player and, after a short delay, an ancient light will emerge from all of them. Ancient lights are vaguely sword-like in design, move at high speeds, and unlike the light shards can actually pass through blocks.
-She may summon an arc consisting of a total of 10 ancient lights. However, as opposed to being released at the same time, each individual ancient light is released in succession; starting at one end and ending at the other.
Throughout the battle small, spherical entities called Illuminant Visions will appear. These simply chase the player, though while they remain active their movement speed increases but knockback resistance decreases. They begin spawning at 80 (100)% health and always ensure that 1 (2) are always active; no more, no less.

Defeat: She falls down, then disappears with a flash of light. Any Illuminant Visions active when she is defeated will instantly despawn.

Drops: A summoning staff that summons Illuminant Visions, a tome, a magic staff, Souls of Light and a sword. In Expert Mode she also drops an accessory that boosts the player's stats whilst in the Hallow.

Hints:
-Moving away from her once she begins attacking can make dodging her projectiles far easier; just don't fly into any portals summoned in her 2nd attack.
-Don't move towards her unless necessary. Moving in her direction can often lead to her stopping to attack right in your path; followed by you taking contact damage which is greater than her projectiles' damage.
-As it takes the Illuminant Visions roughly 3 seconds to respawn, the player could deal with them as they spawn to ensure that they catch the player off-guard later in the fight

Extra Info:
-Of the humanoid bosses, the Lightseeker is one of the only ones to not have some sort of teleportation, dash, or some sort of flight. However, she can still hang in the air when attacking; one of the other traits common among the humanoid bosses.
--Also, unlike the majority of others, she is never actually given a name, only a title.
[Added: 7/8/19]

Appearance: A large bronze-coloured, fairly spherical machine. A small jet of blue flame spouts from its base to give it movement and tubes filled with similarly-coloured blue connect from its body to the large bronze 'arms' either side of the centre. The bronze 'arms' curve around the centre point, designed to look like a laser. All the blue on its body, excluding its flame, glows brightly at certain intervals.

Summon: Use the Arcane Beacon at the surface at night. While the boss will not despawn during the day, a message saying "The beacon's light is too weak to see" will appear if the summoning item is used during the day.

Fought: Hardmode
Specification: Post-Plantera
Special?: No
Progression?: No

Stats:
Health:
-75,000 (115,000) (Engine)
-Invincible (Invincible) (Bronze Sawblade, Bronze Turret)
Attack:
-65 (108) (Melee)
-55 (90) (Laser)
-70 (114) (Bronze Sawblade) (Inflicts Broken Armour - 33 (50)%)
-60 (98) (Missile) (Inflicts On Fire! - 50 (100)%)
-80 (130) (Deathray)
Defence: 20

Attacks:
Phase 1:
Flies near the player; usually trying to stay above them either to the side or directly above. Usually follows a pattern where it fires lasers directly at the player before stopping and using one of its special attacks. The firing speed for the lasers increases as the Engine's health decreases.
-It may fire 4 missiles at the player which home-in on the player. They continue to give chase until they explode after roughly 5 seconds, with the radii of the explosions being roughly 4 blocks.
-It may disappear before re-appearing near to the player and suddenly charging.
-It may stay still, though continues facing the player for 4 seconds. The 5th second will have it stop following the player's movements and, after that 5th second, it will fire a large deathray. The deathray remains active for 3 seconds (and slowly moves in the player's direction).
At 70% and 40% (or, in Expert, at 80%, 50% and 30%), a Bronze Sawblade will be summoned. These follow an AI similar to bats; chasing the player but with slow acceleration and bouncing off of blocks. However, they are faster than bats and, if the player is too far away from them, they are allowed to temporarily pass through blocks. They are affected by knockback and thus can be knocked away from the player, but they cannot be destroyed. If one somehow despawns it is replaced by a new one being summoned by the Engine in its place, so by 40 (30)% there will always be 2 (3) active.
At 20% health it will transition to its 2nd phase.
Phase 2:
In Phase 2 the sawblades remain active, but the Engine's own attack patterns change. Now, as opposed to flying around, it will teleport, attack, and then teleport again. Its attacks in this phase are as follows:
-It may charge at the player after teleporting, similar to how it did in phase 1.
-It may appear, fire 5 lasers at the player in succession, and then fire an arc of 4 (6) lasers roughly half a second later.
-It may summon 3 (4) turrets nearby instead of attacking. These turrets fire 2 lasers at the player, one every 1.5 seconds, and don't despawn until the Engine finishes its next attack; which is usually around 3 seconds after the turrets are spawned.
-It may fire 2-3 (3) missiles at the player that home-in and explode after a few seconds.

Defeat: Explodes into fiery gibs, with a large explosion signalling its defeat.

Drops: A tome that launches cogs, a staff that summons two large cogs near the cursor that rapidly damage enemies, an energy sword, and a stronger clockwork assault rifle. in Expert Mode it also drops the Mechanical Jet Piece; used to upgrade the Mechanical Cart into a mount that works both on and off-rail (flies when off-rail, but is fairly tricky to control).

Hints:
-Dash-granting accessories, like the Shield of Cthulhu and Master Ninja Gear, can be very useful for dodging both its melee and laser attacks.
-Using high-knockback weapons can potentially launch the sawblades far away from the player and, potentially despawn them temporarily. Just make sure not to ricochet them back into your face.
-The sawblades can sometimes get caught in unusual geometry; effectively removing them from the fight.
-The deathray has a long wind-up period that is very different from its other attacks; look out for it.

Extra Info:
-Despite being its overall most powerful attack, the Deathray is only used in Phase 1
--The Deathray also lacks a debuff, whereas some of its other strong attacks have debuffs.
-Despite sharing some visual similarities with the Twins, it is probably closer overall to the Ancient Machine from Qwerty's mod; due to its bronze colouration, some similar projectiles, and its teleportation abilities.
[Added: 10/8/19]

Terminus' NPC can be found in the Cragged Depths once Skeletron has been defeated. Interacting with Terminus will provide 2 options; speak and challenge. Pressing Challenge will allow the player to fight him, whereas pressing Speak will cause him to provide dialogue and to pull out a red book and begin reading it:
-He may quote small portions of Edgar Allan Poe's The Raven.
-He may talk to the player about either The Black Frost or The Cragged Depths.
-He may talk about bosses the player may face in the future, though only in brief phrases and quotes. Some of these bosses include the Lightseeker, Phantasmaw, Void and Light Slimes, Blacksteel, and Terminus himself.

Health: ??? (Cannot take damage from any source)
Attack: ??? (Doesn't attack enemies)
Defence: ??? (As he cannot take damage his defence stat is unknown)

Extra Info:
-Terminus' NPC will disappear when any bosses are active temporarily, closing dialogue boxes in the process.
--Terminus' NPC will also disappear indefinitely upon defeating Terminus' boss fight, so the player might want to hear some of his dialogue before defeating him.

Appearance: A shady figure wearing a red and gold cloak. His entire body is completely black, except for his white eyes. He wears a gold scarf that flows in the wind.

Summon: Find Terminus' NPC near the fountain in the Cragged Depths and speak to him. Select the 'Challenge' option and the NPC will be swapped out with his boss. There are no other ways to summon him. There are, luckily, no real consequences to taking him out of the Cragged Depths, but the fact that it already provides an open area may be beneficial for the player regardless.

Fought: Pre-Hardmode
Specification: Post-Skeletron
Special?: Uncraftable summon, limited fights
Progression?: Yes (Later Bosses)

Stats:
Health: 5,000 (9,500)
Attack:
-40 (80) (Melee)
-35 (72) (Fireball) (Inflicts On Fire! - 100%)
-37 (75) (Red Phantom) (Inflicts Crimson Embers - 20 (50)%)
-45 (88) (Red Impaler) (Inflicts Crimson Embers - 50 (100)%)
Defence: 10

Attacks:
Terminus will attack, disappear, and then re-appear to attack again; with only short breaks inbetween attacks before he teleports in which he is most vulnerable.
-Terminus may appear above the player, dashing downwards until he slams into the ground. This is more commonly used against airborne players.
-Terminus may appear on the ground and dash forwards towards the player, using a clawed hand to slash at the player as he dashes. This is used only when players are close to the ground.
-Terminus may appear near the sides of the arena and open his cloak to release numerous fireballs. They all arc through the air and explode on contact with solid blocks, each travelling a different distance.
At 50% health, Terminus will begin emanating small red particles, and with this also gain a few new attacks.
-Terminus may float into the air near the centre of the arena, releasing large amounts of fireballs in random directions. They act the same as before; arcing through the air until hitting a solid object and exploding.
-Terminus may stand roughly 10 blocks from the player, open his cloak, and launch 4-5 Red Phantoms towards the player in succession. These travel in a straight line but, due to the player's limited mobility and their size, speed, number and frequency, they are hard to dodge.
-Terminus may disappear before 8-10 (10-14) small spikes appear from the ground. Roughly a second later they will burst upwards into larger spikes which remain there for a couple of seconds before disappearing. The smaller spikes deal no damage, but the larger ones will. This attack may only be used twice in succession before Terminus is forced to use a new attack.
-(In Expert Mode, spikes may appear from the ground every few seconds, even outside of the dedicated attack. Their placement is random, though their numbers are set to 1 between 50 and 35%, 2 between 35 and 15%, and 3 under 15%.)

Defeat: Terminus bows, then disappears into a large amount of red and black particles. By the time the particles have cleared, Terminus has disappeared.
-Terminus will no longer appear in the Cragged Depths, meaning he cannot be rematched again under normal means.

Drops: A sword, boomerang, bow, magic staff, and a shield that allows the player to dash through enemies, but doesn't deal damage. Each of these can be upgraded later in the game, though can also be dropped later as well. In Expert Mode he drops the Future Fragment; a material whose description reads; "A small glimpse of what is to come, though the fragment is too small to see what happens clearly."

Hints:
-Fighting it within the confines of the Cragged Depths is alright, as it provides adequate space for the fight and is the sort of arena Terminus' moveset was designed around.
-Terminus can inflict a rather powerful debuff for this early in the game, made worse by the attacks inflicting it being some of his most hard to dodge effectively.
-Distancing yourself from Terminus until after he attacks could be a good method for fighting him, as it provides more time to see the next attack and more time to dodge therefore.
-Playing a more defensive game may be good due to his fast attacks and high damage, combined with teleportation and damaging debuffs. The fight is not under a timer, and the only restrictions are yours and Terminus' health bars.

Extra Info:
-Terminus is one of few non-human bosses to not appear to die or be destroyed after being defeated by the player, simply bowing politely and leaving
--This may, therefore, suggest that Terminus wasn't going all-out due to how calm his exit is
-Terminus' NPC doesn't return after the fight, meaning you only get one round of his loot.
--Luckily, most of his loot is just downgraded versions of later weapons that the player can upgrade then anyways.
-Terminus shares visual similarities with Blackrune; a creature formed from a fragment of the Chaos Doppelganger, who appeared back in Boss Ideas 5. Similarities include the purely-black reflectionless body, the glowing white eyes, and the clawed hands (only visible during some attacks or when he takes out a book)
--However, unlike Blackrune, he appears more sophisticated; even being capable of some rather verbose speech, and having a few books within his possession.
[Added: 10/8/19]

Appearance: A large worm-like creature made of plantlife. Numerous red spikes coat its body; some areas coated more sparsely than others, and those coated heavily often having the green exterior of the worm completely covered over by the red spikes.

Summon: Use the Spiky Pomegranate in the underground jungle. The boss will enrage if taken out of the underground jungle, gaining both speed and damage in the process.

Fought: Hardmode
Specification: Post-Golem
Special?: No
Progression: No

Stats:
[Phase 1]
Health: 350,000 (600,000)
Attack:
-120 (300) (Head Melee) (Inflicts Venom - 50 (100)%/ Inflicts Hypersensitivity 30 (50)%)
-70 (130) (Body Melee, Spiked Ball) (Inflicts Venom - 50 (100)%/ Inflicts Hypersensitivity 0 (25)%)
-50 (80) (Spikes) (Inflicts Venom - 33 (50)%)
Defence:
-50 (Head)
-30 (Body Type I)
-50 +90% Damage Resistance (Body Type II)

[Phase 2]
Health: 20,000 (35,000)
Attack:
-70 (130) (Melee, Spiked Ball) (Inflicts Venom - 50 (100)%/ Inflicts Hypersensitivity 0 (25)%)
Defence: 40

[In Both Phases, damage is increased by 20% whilst outside of the underground jungle]

Attacks:
Phase 1:
The Bloom Worm acts similar to other worm enemies, burrowing through the ground to catch the player. As it burrows it progressively speeds up as its health wanes. It has a few attacks it can use whilst burrowing.
-It may launch barrages of spikes at the player from all its segments. While they are slow, their number and ability to pass through blocks makes them a threat.
-It may launch Spiked Balls from its head which act similarly to how Plantera's act; bouncing around for a while before eventually despawning.
Once the boss' health reaches 0, it will transition to its 2nd phase.
Phase 2:
Its body explodes and the head begins to act on its own. The head is able to fly in this phase, and will begin flying near the player like the EoC. Its attack pattern also changes.
-It will charge at the player up to 3 times in succession, gaining speed both in-between charges and as its health lowers as a general boost.
-After charging, it will throw a spiky ball at the player. They act the same as they did back in phase 1, though persist slightly longer.

Defeat: At the end of Phase 1 most of its body explodes into gibs and damaging spikes. At the end of Phase 2 it explodes into gibs, though doesn't release any additional spikes this time.

Drops: A flail that acts like weapons like the Ball'o'Hurt, but the chain is also able to deal damage. Also drops a sword that launches spikes, a tome that rapidly launches needles that stick into enemies, a gun that acts like the tome but uses ammo instead of mana, and finally drops a Plantera-like sentry. In Expert Mode it also drops the Thorny Bulwark; which deals 2x damage to enemies that deal damage to the player, and also the Thorns of Agony if killed whilst enraged that launches spikes outwards in all directions upon being hit. Both of these can be combined with each other and with other accessories to craft a very powerful accessory.

Hints:
-Piercing weapons, while still very effective, are only about half as effective on this boss due to half of its segments having high damage resistance. The segments with this massive resistance are luckily very obvious; being the ones completely concealed in red spikes.
-All of this boss' attacks inflict Venom, but contact with the boss may inflict Hypersensitivity. This debuff is exceptionally dangerous as it may leave the player open to being Stunned; one of the deadliest debuffs even with its low duration.
-Having a platform far from the ground can be useful, but remember that it follows a similar rule to the Destroyer where it may fly after the player if they are too far away.
-The boss enrages when taken out of the underground jungle, so the best place to take it if you want a fight against its enraged form is the surface; as it provides the most room to move around and dodge its attacks.
-Don't fight it in an enclosed space. The spiky balls it drops, its large body, and many tile-ignoring projectiles make fighting it in an enclosed space a nightmare.

Extra Info:
-The Thorns of Agony accessory it drops is a direct reference to the charm of the same name in the game Hollow Knight, which also provided a similar effect in dealing damage to enemies that have just damaged the player.
-Despite sharing similarities with both the Destroyer and the EoC in its 2 phases, many of the tactics that would work on them actually don't work on the Bloom Worm. Also, the player has to change up their strategy almost entirely between its 2 phases due to how drastically different they are.
-The best way to get its enraged-exclusive accessory in Expert Mode is simply to take it out of the underground jungle while it is already low on health and in its 2nd phase. However, this is technically cheating and isn't particularly showing sportsmanship.
[Added: 10/8/19]

Appearance: A large, purple construction, emanating purple flames from 5 places atop its body and 2 smaller flames; one from each side. A total of 9 'eyes' can be seen on its body, 3 to the left, 3 to the right, and 3 facing outwards from the front. Large purple crystals protrude from numerous parts of its darker-purple body; making claws, spikes, borders, and additional decoration and support. It is held up by 4 small leg-like stands that, despite their size, seem to be holding it steadily.

Summon: Use the Shadowflame Monolith in the Cragged Depths. The boss can be taken anywhere once it has been summoned, however, without any real consequence.

Fought: Chaos Mode
Specifications: ???
Special?: No
Progression: Yes, Later Boss

Stats:
Health:
-1,000,000 x2 (1,650,000 x2) (Head)
-800,000 x4 (1,200,000 x4) (Arms)
-500,000 x4 (750,000 x4) (Flame-Spouting Pillars)
-700,000 x2 (1,000,000 x2) (Legs)
-5,000,000 (7,450,000) (Body)
-3,500,000 (4,800,000) (Core)
-200,000 x6 (300,000 x6) (Dread Embers)
--18,300,000 (27,150,000) (Total)
Attack:
-580 (950) (Melee)
-620 (1000) (Stomp)
-440 (765) (Fist Melee)
-420 (730) (Shockwave)
-400 (700) (Shadowflame Fireball)
-500 (865) (Phantasmal Bomb)
-300 (550) (Bomb Ember)
-415 (723) (Shadowflame Spirit, Flame Pillar)
Defence:
-200 (Pillars, Legs)
-350 (Core, Head, Body, Arms)
-75 (Dread Embers)

[This boss receives a 50% Damage Reduction due to being a Chaos Mode boss]
[All of this boss' attacks inflict Chaotic Ember briefly, as well as Shadowflame for a longer period]
[Heavy Gravity is inflicted upon the player until they player reaches its 3rd phase.]

Attacks:
Phase 1:
All of this boss' segments are divided into their own separate entities, complete with differing stats and attacks, though it all still remains connected to its body. Usually, a segment will have 2 lives each; gaining attacks as it loses those 2 lives and becoming invulnerable once both lives have expired. The Phantasm simply jumps towards the player in this phase while the arms, legs, smaller pillars and head attack. The Arms, Legs, Smaller Pillars and Head are vulnerable in this phase, but not the body.
[Arms]
-Act like the Golem's; firing themselves at the player before retracting. Like the Golem they can be retracted prematurely by attacking them, and their speed increases as their health gets lower.
-In their 2nd life they are slower overall, but can no longer be sent back prematurely when attacked; only slowed down slightly. They release fireballs both while completely retracted and when the arms reach their maximum extension; with the fireballs bouncing around the room also akin to Golem's.
-Once they have been destroyed, they continue releasing fireballs. However, the arms are not launched at the player any more.
[Legs]
-Don't use any proper attacks at the beginning, but if the player makes contact with them at any time they will deal massive damage.
-Once they have lost a life they will begin creating shockwaves whenever the Phantasm lands after jumping. These shockwaves cover an area of roughly 15 blocks outwards left and right, but only about 5 blocks up and 15 blocks down (30x20 in total).
-After depleting both of the legs' lives, they no longer deal their own separate contact damage, and the shockwaves are reduced to only 10 blocks outwards in both directions and 4 blocks up and 8 down (20x12 in total).
[Smaller Pillars]
-The small pillars are sat behind the larger 3 columns of flame and make it harder overall to attack the head in the middle. At the beginning they do nothing.
-Once a down a life they will begin erupting with large columns of flame upwards that deal high damage, as well as releasing shadowflame fireballs that bounce like the Flower of Fire's.
-Once their lives are depleted they release only the bouncing fireballs, but cannot release the large flame pillars anymore.
[Head]
-The head, at the beginning, launches fireballs at the player which bounce off of walls like Golem's.
-However, once taken down to 1 life, it will begin using other attacks. It may launch bursts of 5 bouncing fireballs; 2 left, 2 right and finally 1 directly upwards. It retains the fireball attack from its 1st life as well.
-Once at 0 lives the 5 fireball attack is reduced to 3, and it loses the Golem-esque fireball attack to instead summon Phantasmal Bombs. Phantasmal Bombs are large purple projectiles that are launched towards the player and, upon contact with a solid object, explode violently, releasing smaller embers that are affected by gravity after exploding.
Upon defeating all 6 vulnerable segments, Phantasm transitions to its 2nd phase.
Phase 2:
Its body becomes the new main target, but begins invulnerable until the 3 Dread Embers that appear nearby are destroyed. The Dread Embers are purple, immobile flames that launch ricocheting fireballs at the player as they take damage. Throughout this whole phase Phantasm attacks the player, even while the Dread Embers are still active.
-Along with its jumps, it may now also sometimes teleport above the player, hang in their air for a short time, and then drop downwards onto the ground. It creates a shockwave, much like it does when it jumps, but cannot use other attacks whilst floating in the air.
-It retains its arms' ability to launch ricocheting fireballs and its head and pillars' bouncing fireballs, but loses the original detonator attack.
-It may summon large amounts of Shadowflame Spirits which simply fly either horizontally or vertically, creating large and deadly 'bullet-hells'.
-It may launch up to 5 'waves' of Shadowflame Spirits from its body. 18-30 are released at a time in random directions out from its head. The time between each wave is less than a second.
-It may jump directly upwards and slam down quicker than normal, summoning 4 flame pillars to its left and 4 more to its right. Additionally, 8 bouncing fireballs are launched; 4 either side, all at differing angles.
Once the body has been defeated, it will transition to its 3rd phase; with the head/core breaking away from the crumbling body.
Phase 3:
With the body destroyed, the Phantasm not only loses most of its original attacks, but also the Heavy Gravity debuff.
As with before, 3 Dread Embers are summoned nearby which leave the core invulnerable until all 3 are destroyed. These 3 embers act the same as the ones summoned in its 2nd phase; firing at the player as their health lowered.
Once the 3 embers are defeated they retreat to Phantasm and begin orbiting it. Phantasm's new attacks are independent of their actions.
-The 3 Dread Embers can either launch Shadowflame Spirits, Bouncing Fireballs or Ricocheting Fireballs at the player at any time.
-Phantasm may charge at the player, disappearing by the end of the charge, before re-appearing near the player and releasing 4 Phantasmal Bombs towards the player.
-It may use its 2 bullet-hell attacks from Phase 2 (both the horizontal and vertical moving spirits and the bursts of spirits).
Once the core's health reaches 60% horizontally-moving Shadowflame Spirits slowly begin spawning in by themselves, independent of what the core is doing. The amount that spawn slowly increases as the Phantasm's health wanes.

Defeat: At the beginning of Phase 3 the main body seems to crumble away, slowly disappearing into numerous purple and black particles as it slowly falls apart. Once the core/head is defeated, it instead explodes into a purple explosion. The explosion can deal damage, however, so be careful.

Drops: A blade that fires piercing shadowflame spirit projectiles, a yo-yo, throwing spear, a tome that summons fireballs and another than launches shadowflame spirits, and a ranged weapon that launches shadowflame bombs. Also drops an accessory that boosts the player's defence and allows them to inflict Shadowflame alongside Oiled. In Expert Mode it drops a mount that looks similar to the Phantasm's body that hops to move and can not only attack foes with its extendable fists and fireballs, but also increases the player's defence and damage slightly.

Hints:
-Due to the boss' many projectiles, AoE effects and massive size and damage, it is recommended to make an arena for it as opposed to just using the Cragged Depths' natural geometry. Luckily, a pickaxe capable of breaking the depths' bricks can be obtained at that point in the game.
-Stay away from the Phantasm as much as possible. Its contact damage is exceptionally high and, if the feet aren't yet defeated, it can deal even higher damage.
-Dashes, as usual, are a great help when dealing with its projectiles, phase 2 teleports, and phase 3 charges.

Extra Info:
-Phantasm actually originated from Boss Ideas 6 under the name Phantasmaw. At this time the larger Phase 1 sprite existed, but due to the complexity the fight already entailed and the effort it already demanded, this part was left out.
--A few remnants of its old fight can still be found here; mainly in its Phase 3 form and the use of Dread Embers; still even being in groups of 3 when summoned.
[Added: 10/8/19]

Appearance: A golden, dragon-like monster. It has 3 eyes and strange fangs either side of its jaws. It has 2 claws on each wing and 3-toed feet. It carries a large, greyish sword in its claws.

Summon: Found passively in the underworld once the player is in Hardmode and has defeated at least 1 mechanical boss, but becomes aggressive once attacked by the player or after getting close to the player. It doesn't despawn unless it the player gets too far away from it.

Fought: Hardmode
Specification: Mechanicals
Special?: No summoning item, natural spawn
Progression?: No

Stats:
Health: 27,500 (50,000)
Attack:
-60 (100) (Melee)
-45 (80) (Fireball) (Inflicts On Fire! - 50 (100)%)
Defence: 20

Attacks:
The Ember Beast flies towards the player most of the time, though will fly above the player instead whilst using its fireball attacks. It has 3 different attacking types it can use:
-It can stop briefly before charging at the player, stopping briefly at the end of the charge as well
-It may fly directly above the player, dropping fireballs down towards the player
-It may swoop down in an arc, launching a barrage of fireballs at the player

Defeat: Explodes into a fiery explosion, only dropping the sword as a gib.

Drops: Fiery Scales; materials used to make a few weapons and upgrade a couple of accessories, as well as a sword or a magic tome. In Expert Mode it drops the Infernal Shield which ignites nearby enemies briefly.

Hints:
-As a lot of the fight will involve moving horizontally, a bridge could be useful to avoid having to risk falling into the lava
--Luckily, as the Wall of Flesh is fought prior, the player may already have a bridge set up
-Dashing, due to the sudden burst of horizontal speed, is very effective in this fight

Extra Info:
-For some reason, it doesn't appear to move its wings very much whilst flying. This may be due to the updraft caused by the underworld's heat
-Due to it lacking a summoning item and only appearing in the midst of the mechanical bosses, this boss can be very easily overlooked without any prior knowledge of this boss' existence
-Despite lacking many attacks, this boss is actually a lot more difficult than first expected due to the limited room in which to fight it
[Added: 14/8/19]

Appearance: A large, orangey-golden serpent coated in metal plates. Blue, glassy orbs trail down its segments, designed to appear like eyes. At the front of its body are two large plates connected to the first segment of its body, pointed inwards like a set of jaws.

Summon: Use the Storm Flare at the surface while it is raining. Upon using the item the rain will not expire until the boss despawns, the screen will darken further, and clouds will appear in the foreground that obscure parts of the screen. The boss will chase players across the map, and thus cannot be fled from by simply running away from it.

Fought: Post-Moon Lord
Specification: ???
Special?: No
Progression: No

Stats:
Health: 500,000 (775,000)
Attack:
-200 (350) (Head Melee)
-130 (215) (Body Melee)
-115 (185) (Flare Barrage)
-110 (178) (Lasers, Static Ball) (Inflicts Electrified 20 (40)%)
-300 (500) (Overload) (Inflicts Unstable Energy 50 (65)%)
Defence: 100 +30% Damage Resistance

Attacks:
Spends a lot of time invisible, only appearing occasionally starting from a storm cloud before going invisible again when it reaches another storm cloud, segment by segment. It doesn't often fly towards the player, instead spending a lot of its time above them, moving from cloud to cloud. Its attacks, while fairly limited, are still quite dangerous.
-It may launch numerous flares upwards rapidly while moving. A few seconds later the flares will begin to fall down from the same horizontal positions as where they were originally launched. They ignore blocks and pierce players as they fall.
-It may summon a large ball of electricity from its head that slowly follows the player. This ball releases lasers outwards in all directions, evenly-spaced. Firstly, 4 lasers are fired, then 5, 6, 7, and then finally it disappears with 8 (and a 9 as well in Expert Mode, released just after the 8).
However, the boss also has a special attack, only launched when a 'meter' is filled. Dealing damage, critting the boss, taking damage, time passing and even dealing certain milestones of damage all fill this meter, only signalled by more and more lights filling up across the blue segments on its body.
-Once the meter is filled, numerous large golden fireballs begin falling from the sky at high speed that ignore blocks and deal high damage. After around 10 seconds of this 'overload' barrage, the lights all return to blue on the Flare Wyrm and the meter resets.

Defeat: The Wyrm's segments all begin exploding one-by-one, starting from the tail and making its way to the head. Upon the head exploding, all the loot is dropped. The Wyrm cannot go invisible whilst exploding and, upon finally being completely defeated, the clouds clear, the screen brightens a little again, and the rain stops.

Drops: A flail that acts like the Solar Eruption, a sword that launches flares upon hitting an enemy, and a magic weapon that deals very high damage, but also has a very high mana cost. Also drops Reinforced Plating; used to make armour and a few weapons. In Expert Mode it also drops the Flare Beacon, which drops flares from the sky near the player occasionally.

Hints:
-Piercing weapons, as usual, work well against this boss. However, due to the fact most of its segments will be invisible at any given time, it is less effective against the Flare Worm than most.
-If the player sees it use its flare attack, the best thing to do would be to find a place where flares weren't launched and wait there. The player, if they were correct in their judgement, will be safe.
-Move away from the laser ball once it becomes active. It will chase the player, yes, but it isn't very fast.

Extra Info:
-Originally, this boss was actually two bosses; one with the ability to launch flares and use the special overload attack, and the other having the control over weather and the ability to hide in the clouds. However, to lower the amount of worm bosses, and to keep it more interesting, they were fused into one.
-The points added to the meter are not the same as those displayed on its segments. Numerous points make up one light, and each light has 4 other colours to shift through to reach the whitish colour to signal that segment's part of the meter being 'full'.
-Usually, in Normal Mode, you can defeat it before the meter fills, but in Expert Mode the meter fills faster so the player would likely have to endure at least one 'overload' attack.
-Somehow, despite being bright orange, it can hide itself in grey clouds without too much hassle.
[Added: 11/8/19]

A large fortress found within the centre of the underworld that appears after the Moon Lord has been defeated, eliminating any blocks it overlaps upon summon.

Appearance: The bricks are all dark grey and the whole fortress is filled with powerful enemies and new loot. At the very bottom of the fortress, and therefore at the bottom of the world, lies a large black abyss-like void. Any items that fall in are destroyed, players and normal enemies die instantly, and blocks like silt and water despawn. The blocks cannot be mined until a very powerful pickaxe is obtained far later in the game.

Type: Structure, Mini-Biome
Spawn: Post-Moon Lord, Centre of Underworld
Special: Pseudo-Unbreakable Bricks, Unique Enemies, New Items

Bosses:
Post-Moon Lord:
Slimes of Light and Void* - A duo of slimes; one representing light, and the other darkness.
Blacksteel* - Enchanted metals, infused with a powerful and very active soul.

*Do not need to be fought within the biome, but materials for their summons are found there

Enemies:
Post-Moon Lord:
-Void Creeper, Void Harpy, Abyss Slime, Abyssal Dribbler, Void Wanderer, Void Mimic (Mini-Boss), Void Paladin (Mini-Boss)
Chaos Mode:
-Blackrune Apparition, Blackrune (Mini-Boss)

Items:
Post-Moon Lord:
-Void Essence (Creeper, Harpy, Slime, Dribbler, Wanderer, Paladin), Void Gel (Slime), Abyssal Plating (Paladin), Shadehammer (Paladin), Abyss Claw (Mimic), Black Hole Staff (Mimic), Void Pendant (Mimic), Void Ripper (Mimic), Void Slime Staff (Mimic)
Chaos Mode:
-Black Rune (Blackrune Apparition, Blackrune), Eye of Ages Past (Blackrune)

Extra Info:
-Due to the fact that it destroys anything else that was in its place when spawned; chests, drops, and even other biomes from other mods may be wiped out once the Void is spawned.
-The pit seen at the bottom of the biome, while it cannot be destroyed, can still be bricked over to avoid accidentally falling in and dying
-The player can be teleported into the instant-kill pit if it isn't covered over. While it is unlikely, it technically can happen
-Many of the enemies seen within the Void are twisted, stronger versions of vanilla enemies like Harpies, Slimes, Dribblers, Mimics, Paladins and Zombies. While they look and act similar, their designs are often twisted; sometimes even beyond recognition.
-Blackrune, the mini-boss that appears in the Void after Chaos Mode is activated, is a reference to Blackrune from Boss Ideas 5 (and in fact may just be him back from the grave...)
-Destroying the Void does not remove the pit at the bottom, nor is there any other way of getting rid of it.
-The Void also serves another purpose outside of providing progression materials in that it rather evenly divides the underworld for another biome to be spawned within it.
[Added: 12/8/19]

Appearance: Two large slimes. The light slime is a light blue slime with strange markings in its body and, at its centre, energies circling each other. The void slime, on the other hand, has a seemingly-empty interior outside of a large number of teeth-like spikes that point inwards towards its centre. What little is visible of the slime itself is purple; the same colour as the spikes.

Summon: Use the Gelatinous Duality anywhere in the world. Both slimes are summoned at the same time and do not despawn unless the player gets too far away or dies, like with most bosses.

Fought: Post-Moon Lord
Specifications: ???
Special?: No
Progression: Yes, Later Bosses

Stats:
[Void Slime]
Health: 650,000 (1,000,000)
Attack:
-165 (270) (Melee)
-140 (220) (Void Fury)
-150 (238) (Black Hole Bomb)
Defence:
-60 (Normal)
-500 +90% Damage Resistance (Whilst under 50% health while Light Slime is above 50%)

[All of the Void Slime's attacks have a 25 (40)% chance to inflict Chaotic Ember]

[Light Slime]
Health: 600,000 (900,000)
Attack:
-190 (315) (Melee)
-150 (238) (Light Blast)
-165 (270) (Shattering Light)
Defence:
-50 (Normal)
-500 +90% Damage Resistance (Whilst under 50% health while Void Slime is above 50%)

[All of the Light Slime's attacks have a 25 (40)% chance to inflict Unstable Energy]

[Misc. Attacks]
-145 (229) (Light Fury)
-150 (238) (Void Light)
-220 (350) (Black Hole's Light)

Attacks:
Phase 1:
Both slimes have similar ways of moving. They rapidly hop towards the player and are able to perform two types of 'super jump', based on the biome mimics. One is a series of 3 hops, each gaining additional speed, distance, and height, and the other has it jump above the player and slam down into the ground at high speeds. Their jump height and movement speed increase as their health gets lower. Like King Slime, they can teleport near the player if they detect they are too far away, but can also pass through blocks as well if they consider it necessary.
-The Void Slime may launch large clusters of Void Fury projectiles at the player. As they travel they slow down, then speed up again, and repeat this until they despawn.
-The Void Slime's other ranged attack has it launch a large amount of 'Black Hole' projectiles in random directions around itself that eventually stop midair. After a while they explode into what appear to me miniature black holes; dealing damage on contact.
-The Light Slime may fire shots in very tightly packed arcs of 5, followed almost immediately by less tightly packed arcs of 3.
-The Light Slime's other attack is similar to the Void Slime's Black Hole Bomb; launching numerous projectiles randomly which hang in the air. However, when they explode, instead of creating large stationary damaging projectiles they instead release glowing light-blue crystals outwards in random directions, usually 3 per projectile.
At 50% health, they will transition to their 2nd phases. If only one is transformed, the one transformed gains massive defence boosts until the other is also transformed.
Phase 2:
Both slimes gain large pairs of wings which they now use to fly in short bursts. The Light Slime's are feathery while the Void Slime's are more draconic and rugged.
-Their jumps now cover more vertical distance. Their triple-jump now has the 3rd jump end with a dash towards the player just before falling, and the attack where they fly above the player and fall down is much faster. They can also now simply jump and dash at the player, or fly in the air, launch an attack or two, and fall down again above the player.
-If certain projectiles collide in this phase, they will change into new types of projectiles with different properties.
--If the Light Blast and Void Fury projectiles collide they create a 'Light's Fury', which acts like the Void Fury, but also releases a damaging trail as it moves that lingers briefly.
--If the Black Hole Bomb and Light Blast or Shattering Light and Void Fury attacks merge they create a 'Void Light', which both leaves an explosion where it detonates as well as releasing small crystalised fragments outwards.
--If a Black Hole Bomb and Shattering Light somehow meet, they create a 'Black Hole's Light', which acts like a fusion of the Void Light and Light's Fury. It creates a massive explosion, launches 8 fragments outwards, and all 8 fragments leave damaging trails briefly as they move.
(In Expert Mode, if one is going to use Black Hole Bomb and the other Shattering Light, it is nearly guaranteed they will purposely launch 1 each to the same location; causing them to merge into a Black Hole's Light.)
(In Expert Mode exclusively, when one is defeated while the other is still alive, it will go into a 'Frenzy' state. It moves and attacks a lot faster, and has a 100% chance to inflict its debuff. This state remains until it is defeated as well and the fight ends.)

Defeat: They explode into small globules of gel and eventually disappear. The globules are just gibs and cannot affect the player in any way.

Drops: Both slimes drop their own individual loot, as opposed to sharing like most multi-part bosses.
-The Void Slime drops Void Gel; a material, a sword, a gun, a throwing spear, a throwing boomerang-esque weapon, and a tome. In Expert Mode it drops a shield that boosts some defensive stats, but also revives the player once after death with a boost to damage.
-The Light Slime drops Radiant Gel; a material, a sword, a bow, a summoner staff, a sentry staff, and a magic staff. In Expert Mode it drops a shield that boosts some offensive stats, but also revives the player once after death with a boost to defence.
-Which ever one is defeated last will drop Elysian Cores; materials used to make a large amount of late-game items.

Hints:
-Dashing away from their 'slam-down' attack is the best way to avoid it. Also falling as it falls can also provide more time to move from underneath it.
-It is probably best to keep their HP fairly close to each other. This is because of the defence boosts when one isn't in Phase 2 and, in Expert Mode, the frenzy mode one will enter if the other is defeated
-If a Black Hole's Light is ever summoned (or, in Expert Mode, when they are summoned), move away from it. It gives the player more time to dodge and also means they won't have to work around the explosion as much as they would if they were close. The explosion is also deceptively large.
-While their debuffs do not last long, they are both extremely dangerous debuffs.

Extra Info:
-The Chaotic Ember debuff inflicted by the Void Slime was originally only exclusive to the Chaos Doppelganger and Chaos Mode bosses. However, strangely, the Void Slime now has access to it... what could this mean...?
-Despite being the Light Slime, the Light Slime is actually closer to the colours of the Soul of Flight, and has an animation closest to the Soul of Might. The main reason is that pink mainly didn't suit the Light Slime well; the same problem as with the Lightseeker post-alteration.
-Despite being the Light Slime, and thus the one most associated with good, it deals more damage than the Void Slime, which seems to be the defensive counterpart of the two.
--This could be a reference to Neoradiance: a large, winged slime from Boss Ideas 4 who was also known for its deceptively high damage output.
[Added: 14/8/19]

Appearance: A person cloaked in a purple and gold robe, hunched over. He is found in the Cragged Depths quite late into the Post-Moon Lord game next to the fountain, though before he can spawn both Terminus and The Slimes of Light and Void must be defeated. Their entire body is concealed, with the head hooded over and facing toward the background, away from the player. The figure is cloaked in darkness at all times, and their cloak a de-saturated purple.
When interacted with, he will begin speaking to the player, though his dialogue will restart if interrupted by cancelling his dialogue.
-He will talk about the player's quest, and about the bosses they have encountered and defeated (based mostly on progression-based bosses), before eventually beginning to talk about his plan of 'shaping the world anew'.
-After saying he will reshape the world the hood of his cloak drops back and reveals a shadowy head, and it continues by saying that he needs power from the void to complete this. Eventually he ends his monologue with "But, when it all comes down to it, I guess you could consider me the villain after all..."
After this, the figure is revealed as Void Terminus, and his fight will begin.

Health: ??? (Cannot take damage from any source)
Attack: ??? (Doesn't attack enemies)
Defence: ??? (As he cannot take damage his defence stat is unknown)

Extra Info:
-The reveal of it being Void Terminus is, to an extent, fairly obvious to those who remember the original battle as this figure has a similarly-designed cloak and stood in the exact same place. However, due to how long ago the fight against Terminus was (Late Pre-Hardmode vs Late Post-Moon Lord) it is possible most had forgotten how he looked.
-The NPC stops appearing after defeating Void Terminus at least once. However, his 'encore' fight can still be rematched even after the NPC stops appearing.
[Added: 14/8/19]

"But, when it all comes down to it, I guess you could consider me the villain after all..."

Appearance: Void Terminus is the empowered form of Terminus, the 4th boss seen on this page, apparently having infused himself with energies from the Void. His cloak has changed from a red to a purple and how has a hood. Black energy radiates from his body in small wisps and a purple barrier of Void energy surrounds him throughout the entire battle.

Summon: Void Terminus has two ways of summoning him. Both times will have the sky darken, even during the day, and both follow the same rules where they only despawn when they have no targets left nearby.
-The main way of summoning Void Terminus is to interact with the NPC found within the Cragged Depths next to the fountain. After a few lines of dialogue Void Terminus will be revealed and the fight will begin.
-After being defeated at least once, it can only be summoned by using an Old Chaos-Caller. However, these fights will instead use his 'encore' stats instead of his original ones.

Fought: Post-Moon Lord (Normal), Chaos Mode (Encore)
Specification: ???
Special?: No summoning item (Normal-only)
Progression: Chaos Mode Transition

Stats:
Health:
-5,000,000 (8,000,000) / 8,250,000 (12,000,000) (Terminus)
-1,000,000 (1,450,000) / 1,500,000 (2,000,000) (Void Barrier)
-50,000 (75,000) / 100,000 (135,000) (Past Reflections)
Attack:
-350 (530) / 500 (750) (Terminus Melee, Barrier Detonations, Reflection Melee)
-300 (456) / 445 (632) (Chaotic Fury, Void Spines)
-310 (468) / 458 (650) (Shadowflame Meteor, Shadowflame Lashes) (Inflicts Shadowflame - 100%)
-330 (500) / 480 (714) (Ichorian Pulse) (Inflicts Ichorian Spark - 100%)
-300 (456) / 458 (650) (Crimson Maws, Crimson Flurry) (Inflicts Crimson Embers - 100%)
-400 (600) / 600 (900) (Chaos Dustcutter) (Inflicts Withered Armour - 100%/ Unstable Energy - 100%)
Defence:
-250 (Terminus)
-450 +10% Damage Resistance (Void Barrier)
-0 (Past Reflections)

[Group 1: Normal Stats, Group 2: Encore Stats]
[All of this boss' attacks inflict Chaotic Ember]

Attacks:
Phase 1:
In this phase Terminus will attack, teleport, and attack again; repeating this cycle. He isn't particularly mobile outside of the teleports at first, however, as he stays still most of the time when attacking.
5 Void Barriers surround him that leave Terminus immune to damage. These need to be destroyed to progress to the next phase.
Terminus' attacks are as follows:
-He may launch large quantities of shadowflame fireballs up into the sky from under his cloak. A few seconds later these begin raining down at high speeds, exploding violently on contact with solid objects. After a few seconds, they will stop falling.
-He may teleport near the player, open his cloak, and unleash a large quantity of rapidly-flailing shadowflame tendrils that lash out at nearby players. He repositions himself up to 3 times during this attack with teleports before ending the attack.
-He may release a large pulse of Ichorian energy in a large patterned blast around himself. A second later, smaller versions of this blast begin detonating in random places nearby, usually only signalled by the appearance of small golden orbs before they detonate. After a couple of seconds these smaller explosions stop spawning.
As more of the Void Barriers are destroyed, Terminus' attacking speed will increase. After 3 are destroyed the number of projectiles per attack also increases by a decent amount.
Once all 5 are destroyed he moves to his 2nd phase.
Phase 2:
Terminus may now move without teleportation whilst attacking, and broadens his attack roster. He retains his attacks from before, but also gains a few new ones as well.
-He may summon large clusters of Past Reflections near himself which chase the player. They are fairly large black projectiles with white grinning faces on them that chase the player through a series of rapid but short-ranged charges. They continue chasing the player until destroyed, even through other attacks.
-He may release a large barrage of black 'Chaotic Fury' projectiles. These move very fast, but also arc slightly as they travel despite being fired directly at the player. Due to their number and their randomised arcs they actually cover quite a large area both in length and width, even if inaccurate.
-He may stay still, floating in-place, before colossal black spikes begin jutting out of him in random places. They stay out for less than a second before retracting and up to 3 'waves' of spikes can occur before the attack ends. The spikes are roughly 10 (12) blocks long and only about a block wide each, but their speed and his tendency to teleport next to the player before using it make it dangerous.
-He may launch a large barrage of Crimson Maw projectiles at the player. They have no special properties outside of their ability to pass through blocks, but their number and the fact he continuously follows the player's movements while summoning them can be troublesome.
-He may release a 'flurry' of these Crimson Maw projectiles, usually 10-12 in an inaccurate burst in the player's general direction. Up to 4 flurries can be used in succession, with very little time in between.
Once down to 40 (55)% health, he transitions into his 3rd phase, signalled by him letting out an echoing roar and a large pulse of darkness.
Phase 3:
He gets faster, gains new attacks, and adds more projectiles to some of the attacks from previous phases. He also begins, sometimes, using a new weapon: a sword called the Chaos Dustcutter.
-Crimson Maw projectiles may sometimes stop and re-adjust their trajectory and 'charge' at the player again. This can only happen once per projectile, however, and isn't guaranteed to happen.
-He may use the Chaos Dustcutter in either a horizontal dash or a downward thrust, depending on whether he was beside or above the player when the attack started. The dash is extremely fast and the thrust fast, but not quite as fast.
-He may charge at the player up to 3 times in succession with the Dustcutter, slashing at the player when he gets close or at the end of the charge.
-He may charge at the player with the Dustcutter, then at the end of the charge launch an arc of 3 Crimson Maws at the player.

Defeat: Lets out another echoing roar like at the beginning of Phase 3, before a pillar of purple-ish shadows surrounds Terminus. A few seconds later, Terminus will be completely gone and the pillar will fade away as well.
-The Cloaked Figure NPC used to trigger the 'normal' version of this fight may no longer spawn
-If not already active, the world will transition into Chaos Mode.

Drops: Always drops a pickaxe capable of mining the Cragged Depths and void, and large quantities of Chaotic Essence; a material present in the majority of Chaos Mode items. His other drops include 2 swords, a yo-yo, boomerang, throwing spear, 2 tomes, a magic staff, a summoner staff, a gun, bow and rocket launcher. In Expert Mode it drops cloak that grants a long-ranged dash that provides immunity whilst dashing, but has a rather lengthy cooldown.
In the Encore fight it will drop 3 weapons at a time instead of 2, he drops more Chaotic Essence, and the pickaxe is now only a random drop as opposed to a guaranteed one.

Hints:
-Do not attempt to fight Void Terminus within the confines of the Cragged Depths. Unlike his original fight, which was tailored to fit the room and the player's pre-hardmode gear, this fight uses a lot of projectiles, summons, and fast movements.
-Many of his attacks inflict 2 debuffs at a time and, in some cases, even three. They can stack very easily and thus, if inflicted with some of the more detrimental debuffs (like Unstable Energy, Ichorian Spark, and Withered Armour) the player should go on the defensive until the debuffs expire.
-Destroy any Past Reflections he summons. They may seem rather unthreatening with their lack of projectiles, but their contact damage and sheer numbers when summoned make them surprisingly dangerous
-While not always possible, attempt to destroy the Void Barriers at around the same times as each other. He speeds up with each one destroyed and even gains additional projectiles when 1 or 2 are left. Defeating all 5 at roughly the same time (like with AoE damage) may skip these boosts and move directly into phase 2 without phase 1 getting even harder.
-If you didn't get the weapon you wanted first-time around, gather some Chaos Mode materials before you try to face him again. His 'Encore' stat boosts make him a reasonably powerful Chaos Mode boss which, if only bringing Post-Moon Lord items, can feel rather unfair.
-His sword attacks seen in later phases deal high damage and can inflict some rather deadly debuffs. If you see him draw his sword at any time, prepare to dodge.

Extra Info:
-Like with most major bosses, Terminus' fight has been re-designed a few times. Originally, he was going to simply mimic his original fight before kicking into his more powerful form, there was also an idea to force the player to destroy his barrier while Terminus himself actually only had low health, and another idea was to have him shift into a larger and more intimidating form mid-fight.
-- The most developed of all the rejected designs was Terminus Apothesis; which would have had Terminus fuse with a light energy and use both light and void attacks in the same fight. However, due to an overall lack of interesting attacks, it was scrapped.
-Terminus is able to inflict a large assortment of debuffs throughout this fight. Chaotic Ember, Shadowflame, Ichorian Spark, Crimson Embers, Unstable Energy and Withered Armour. Therefore, the maximum DPS he can deal via debuffs is 108 DPS and the defence reductions are -50% defence, -50 defence, and the removal of additional damage reduction (like the Worm Scarf and Endurance Potion)
--Therefore, a player with 100 defence can be reduced to 0 if inflicted with Withered Armour, then Ichorian Spark and Unstable Energy afterwards.
[Added: 14/8/19]

Appearance: A humanoid fungal warrior. Her outfit is a mixture of a purple-ish indigo and dark blue, with a small pouch around her waist only visible when her arms aren't at her sides. Her face is mostly obscured by a large fungal cap that is angled down over her face with dark blue 'hair' flowing down from it.

Summon: Use the Scroll of Shroom in the surface mushroom biome. The boss only despawns if there are no nearby players; seemingly unaffected by leaving the biome.

Fought: Post-Moon Lord
Specification: ???
Special?: No
Progression?: No

Stats:
Health:
-100,000 (135,000) (Mycel)
-1,000 (1,500) (Spores)
Attack:
-75 (140) (Melee)
-70 (132) (Spores, Mycelium Chakram)
-60 (120) (Spore Cloud)
Defence:
-15 (Mycel)
-0 (Spores)

Attacks:
Mycel uses a similar attack style to a lot of other humanoid bosses; attacking with teleports in-between. However, Mycel actually spends very little time visible and vulnerable, disappearing again very quickly after attacks. It has a few different attacks to use:
-She may appear near the player and charge at them with a katana-like sword, disappearing after the charge ends
-She may appear above the player and perform a downward thrust, disappearing after little more than a second again
-She may appear and throw a Mycelium Chakram at the player, which chases the player for a few seconds until it either expires or hits the player, even persisting into the next attack.
-She may take some spores from her pouch and throw a cluster of them towards the player. They slowly descend, following the player's horizontal position
-She may take spores from her pouch, though just as they are thrown slashes at them a few times with her sword. They then explode into a very large spore cloud which lingers in-place for a lengthy amount of time.

Defeat: Mycel's cap is knocked off, revealing her face. In retaliation she draws her sword but it breaks, so she flees; picking her cap off the floor as she does so.

Drops: Drops her sword, the Mycelium Chakram, and a Spore Bag that releases spore clouds when used. Also drops an accessory in Expert Mode that releases an explosion of spores upon taking damage.

Hints:
-The player will need to attack Mycel while she attacks, as she disappears at the end of every attack
-Many of her attacks can persist until the next attack begins, so the player will need to keep their eye out for the spores and Chakram quite often
-Dealing with the spores as they fall, with piercing and AoE weapons, can effectively neutralise the falling spore attack at the cost of a few seconds. It may be worthwhile, especially if there are lots of them active at once.

Extra Info:
-Mycel's original fight was planned to actually consist of numerous enemies at once, with Mycel being the leader and between 2-3 other warriors assisting her. However, due to another later boss utilising that idea and having overall more importance, plus the overlapping attacks already being difficult to deal with, the idea was scrapped
--So, in turn, more of Mycel's own attacks were allowed to linger around for an additional amount of time so that some attacks could overlap to increase difficulty.
-Mycel lacks a Ranged or Summoner drop, which means that the fight could actually be skipped in certain playthroughs if the player wasn't going out of their way to fight all bosses
--The fight, overall, can be easily overlooked on most runs due to the surface mushroom biome normally being a ranger-specific necessity, the fight's lack-of narrative importance, and how early into the Post-Moon Lord the game she can be fought where the player would probably be dealing with far larger and more intimidating bosses already at that point.
[Added: 15/8/19]

A frosted wasteland, created by a powerful arctic force that froze over half of the underworld.

Appearance: Ash blocks are replaced with grey ice, lava with icy water, and Hellstone with Cryostone Ore. Icy Water deals damage when the player makes contact and inflicts Chilled for a few seconds. It cannot be overridden with the lava charm, and needs its own immunities to avoid taking damage from it.

Type: Structure, Mini-Biome
Spawn: Post-Moon Lord, Dungeon-Side of Underworld
Special: Unique Enemies, New Items

Bosses:
Post-Moon Lord:
Blacksteel* - A chilled core of ice surrounded by an enchanted, strange black metal.
Chaos Mode:
Alynea - An Ice Mage who lives within the depths of the Permafrost

Enemies:
Post-Moon Lord:
-Cryowisp (Normal), Frostwing (Normal), Arctic Wolf (Normal), Frost Gel (Normal), Frost Amarok (Mini-Boss), Frost Wyvern (Mini-Boss), Cryomimic (Mini-Boss)

Items:
Post-Moon Lord:
-Cryosteel (Cryowisp, Frostwing, Frost Wyvern), Frost Pelt (Arctic Wolf, Frost Amarok), Chilled Gel (Frost Gel), Hunter's Bow/ Frost Grapple/ Great Wolf's Tooth (Frost Amarok), Wyvern Wing/ Dragon's Breath (Frost Wyvern), Icicle Spear/ Cryowisp Staff/ Frosterang/ Ice Wyvern Staff/ Frost Cloak (Cryomimic)

Extra Info:
-The Permafrost will always be on the same side of the world as the Ice Biome and Dungeon, separated from the rest of the underworld via the Void fortress that appears in the centre.
-If the player has mined the Hellstone back before the Moon Lord was defeated, the potential maximum amount of Cryostone the player can obtain will decrease.
-The reason for this half of the underworld freezing over may be due to Alynea; a resident ice mage who can only be found in the Permafrost
-The Permafrost may be a reference to the term 'Hell froze over'; referring to an extremely unlikely event and the likelihood of it taking place
[Added: 14/8/19]

Appearance: A glowing blue core surrounded by chunks of strange, black metal. The core glows blue and has two small wisps of light orbiting the centre of the core. The metal surrounding the core seems to mainly be random chunks, though some of the more hollow pieces seem to reflect the blue glow from the core.

Summon: Use the Chilled Energy Core in the snow biome, underground snow biome, or the Permafrost. The boss will enrage if taken out of these biomes; gaining additional movement speed and damage.

Fought: Post-Moon Lord
Specification?: ???
Special?: No
Progression?: No

Stats:
Health:
-800,000 (1,150,000) (Blacksteel)
-750,000 (1,100,000) (Blacksteel Wyrm)
Attack:
-140 (240) (Blacksteel Melee)
-125 (200) (Frost Bolt)
-200 (350) (Wyrm Head Melee)
-150 (258) (Wyrm Body Melee)
Defence:
-200 (Blacksteel)
-400 (99,999) (Wyrm Head)
-99,999 (Wyrm Body)

[All attacks have a 25 (33)% chance to inflict Chilled]

Attacks:
Blacksteel itself will simply fly around, performing one of a few different attacks
-It may float horizontally to the player for a few seconds, and then charge at them
-It may perform a series of chain-dashes towards the player, gaining speed with each dash, until it stops
-It may stay in-place and launch Frost Bolt projectiles out in random directions. These pass through blocks and pierce players.
At certain intervals of health, Blacksteel will summon 'Blacksteel Wyrms' to assist it. They are summoned at 80%, 60%, 40% and 20% (Or, in Expert Mode, 85%, 70%, 55%, 40%, 25% and 10%)
Blacksteel Wyrms simply fly towards the player, though occasionally may gain speed temporarily. The speed boosts are more common when flying in the same direction for a long time, or when the player is far away from them. They are only 11 segments long but, due to how many can spawn, are a threat.

Defeat: Blacksteel explodes, leaving behind gibs of the metal around its core, but not of the core itself. The Wyrms all flee once Blacksteel is defeated, burrowing downwards rapidly and ignoring players.

Drops: A flail, a high-speed throwing knife, 2 magic staffs and a gun. Drops Energised Frost Cores that are used as a material to make armour. Also drops an accessory in Expert Mode that allows the player to encapsulate themselves in a shield of metal and ice; regenerating health and significantly boosting defence at the cost of not being able to move or attack when it is in effect.

Hints:
-Moving in one direction constantly to distance yourself from the worms doesn't often prove effective as they gain speed if the player keeps moving in one direction or gets too far away. This speed boost has no cap and so even things like powered rails and a mechanical cart don't prove too effective.
-Fighting it at the surface ice biome is best due to the amount of room it provides
-Do not try to destroy the worms in Expert Mode, their defences are too high to really damage them significantly with anything
--However, in Normal Mode, dealing with the worms may prove useful to stop the fight from becoming too overwhelming

Extra Info:
-Technically speaking, the Wyrms are animated... but both frames of animation are identical. This was mainly to stop the animation from breaking as, with only 1 frame, it would often gain a black or white background; though adding a 2nd frame stops this
-Originally, there was a plan to have the fight occur the opposite way round; with the Wyrm being the main boss and Blacksteels being summoned as the fight progressed, though it was scrapped due to how easy it appeared to dodge the smaller minions compared to the Wyrm's massive hitbox.
--Another plan was for Blacksteel to be able to protect itself much like a Granite Elemental when attacked. However, due to how much this appeared to upset the pacing of the fight, this was also scrapped as an idea.
-Blacksteel's name is rather self-explanatory; being predominantly surrounded in a black metal. Even the Wyrms are self-explanatory, as they are just worms coated in the same black metal.
[Added: 16/8/19]

Appearance: A humanoid mage coated in a black and blue hooded cloak. Most of the body and sleeves of the coat are blue, with the fur trims and decoration all sharing a reflective black. Her hair is blue; the same blue as the one on the coat.

Summon: Use the Absolute Zero in the Permafrost. While the boss can be taken anywhere after being spawned, her stats will increase by varying amounts in other biomes:
-In the Snow or Underground Snow biomes, she gains 1.2x damage and 2x defence
-In Most other biomes, she gains 1.5x damage and 3x defence
-In the Underworld, Desert, or Underground Desert, she gains 2x damage and 5x defence

Fought: Chaos Mode
Specification: ???
Special?: No
Progression?: No

Stats:
Health:
-5,000,000 (8,450,000) (Alynea)
-1,000,000 (1,500,000) (Frost Barrier)
-1 (1) (Reflective Ice Chunk)
Attack:
-450 (780) (Melee) (Inflicts Chilled - 33 (50)%/ Inflicts Frozen - 20 (50)%)
-460 (794) (Frost Flare, Arcane Ice) (Inflicts Chilled - 25 (40)%)
-500 (865) (Cryomissile) (Inflicts Chilled - 65 (100)%)
-440 (765) (Icicle Bombardment) (Inflicts Chilled - 25 (40)%)
-600 (1000) (Absolute Zero, Permafreeze) (Inflicts Chilled - 65 (100)%)
-350 (615) (Arctic Wind) (Inflicts Chilled - 25 (40)%)
-10 (15) (Frost Pelter) (Provides no invulnerability frames and ignores defence)
-375 (665) (Ice Spike) (Inflicts Chilled - 25 (40)%)
Defence:
-200 (Alynea)
-750 +15% Damage Resistance (Frost Barrier, Ice Chunk)

[All of this boss' attacks inflict Frostburn]

Attacks:
Phase 1:
Alynea often floats in place, attacks, and then teleports elsewhere at attack again. However, instead of a regular teleport, she will disappear and leave in her place a 'Reflective Ice Chunk' that looks like her, but paler. Destroying this chunk will cause her to re-appear elsewhere and continue as normal, but leaving the chunk will cause it to eventually explode on its own, releasing massive quantities of Ice Spikes out in random directions, with Alynea appearing again as it explodes.
Alynea's attacks are as follows:
-She may launch a small blue flare upwards. Shortly after large amounts of snowflakes begin descending from the sky around the arena, and continue to do so for a while to the point where they may persist into the next attack or two.
-She may rapidly launch Arcane Ice bolts at the player. They accelerate rapidly as they travel and pass through blocks, but otherwise don't have any special properties. After launching between 10-15 at the player she stops the attack.
-She may launch numerous arcs of Arcane Ice bolts at the player. The first arc is 3, then 6, then 5, then 4, then ending with 3 again. The projectiles accelerate as they travel, like before.
-She may summon between 3-4 large ice crystals near herself, then unleash them all. They all chase the player before exploding into Ice Spikes, usually 7-8 spikes each in random directions. They explode at random times, unaffected by blocks or the player's actions.
Once at 60 (70)% health, she enters her 2nd phase.
Phase 2:
Alynea surrounds herself in a frost barrier and begins to regenerate health. Her attacks are the same, but she doesn't leave Ice Chunks as she teleports around with the barrier. She can only regenerate back up to 80 (85)% health before she stops, though this phase continues until the barrier is broken. If broken prematurely she will regain all of the health she can at once.
Phase 3:
Alynea appears angry, then goes back to attacking. She summons Ice Chunks as she teleports again, as expected, and retains her previous attacks. However, she also gains new attacks as well:
-She may summon anywhere from 20-30 icicles in the sky near the player and herself, let them linger in the air for only around half a second, then drop all of them downwards. They will become 'stuck' in blocks briefly, where they can still deal damage, before disappearing a few seconds later.
-She may summon an 'arctic wind'; a large white cloud of mist that lingers in-place for an extended period of time. It often lingers for up to 30 seconds before beginning to disappear, making it effective area denial.
-Her most powerful new attack is the Absolute Zero. She summons all her energy and creates a large white explosion around herself, dealing excessive damage if the player is too close. This explosion will often also launch out Arcane Ice bolts and Ice Spikes in random directions, but they deal their regular damage.
At 30 (35)% health she transitions to her 4th phase
Phase 4:
She re-enters her frost barrier, thus losing the ability to summon Ice Chunks again in exchange for a shield to regenerate behind. She retains her previous attacks, excluding the Absolute Zero, and regenerates health rapidly back up to 75 (80)%.
Once the shield is destroyed again she will transition to her next phase.
Phase 5:
Appears angry again for a second, then goes back to attacking. The Ice Chunks and the Absolute Zero attack return once more, though she also gains two more powerful attacks.
-She may begin summoning lots of small thin spikes of ice called Frost Pelters. These are rapidly fired in the player's general direction, but are inaccurate and cannot pass through blocks. However, their sheer number and ability to ignore immunity frames and defence make them very threatening regardless.
-Her final new attack is the Permafreeze. She will charge it up for a few seconds, before a blizzard-like wind deals high damage to any players even remotely nearby. The only way to 'dodge' the attack is to deal damage to Alynea while she is charging it, where she will lose concentration and not be able to launch the attack.
At 30% health she transitions to her 6th phase
Phase 6:
Regains the frost barrier, but doesn't lose the Ice Chunks this time. While she cannot use either the Permafrost or Absolute Zero attacks in this state, she rapidly regenerates health back to 100% regardless of difficulty. Whenever she teleports 3 Ice Chunks are summoned; one where she was and the other two at random points between where she teleported from and to.
Once the barrier is defeated again, she transitions to her 7th and final main phase.
Phase 7:
Regains all of her attacks from before, summons 3 Ice Chunks with every teleports like in the last phase, and overall seems to attack a little faster.
At 10% health she transitions to her final phase.
Phase 8:
Loses all of her attacks except the Permafrost; which she tries to use continuously. Cannot summon any more Ice Chunks, either. If her Permafrost attack is interrupted she will teleport immediately without hesitation in this phase. Her teleports will always be fairly close to the player, however.

Defeat: Lets out a yell, then begins rapidly teleporting around, creating Ice Chunks. She teleports faster and faster as the Ice Chunks begin to crumble away instead of exploding like they would have during the fight, with her eventually teleporting away and ending the fight.
-Alynea's NPC can move in, who sells a few different weapons, some materials, and a few accessories based on the biomes her house is located in

Drops: 2 swords, a spear, throwing knives, a snowflake chakram, 2 tomes, a magic staff, a summoner staff, a gun or a bow. She may also drop an accessory that allows the player to inflict Frostburn and deal additional damage to targets inflicted with it, as well as a pair of wings. In Expert Mode she drops the Archmage's Pendant, which provides immunity to Chilled, Frozen, Frostburn, and the Permafrost's icy waters.

Hints:
-It is recommended the player destroys the Ice Chunks as the spikes they spawn otherwise may provide problems during the next attack as it provides more projectiles to deal with.
-Fast-firing and high-velocity weapons prove effective in this fight, as they are both able to deal with the Ice Chunks quickly and also stop Alynea from being able to use Permafrost as freely
-The simple fact her health seems low for a Chaos Mode boss should be a subtle hint that she has some sort of regenerative property, which she does.
-Carving out a large part of the Permafrost could be useful for the fight, as taking her to any other biome will simply enrage her and make her stronger. If not possible for any reason, the next best place is the surface snow biome because of the room it provides combined with the minimal increases to her stats.

Extra Info:
-For whatever reason, her NPC appears to be smaller than her boss sprite. This may just be done, however, for NPC size consistency for the NPC, and a larger hitbox for the boss.
-Alynea's name is, much like with the King's minions back in Boss Ideas 5, not a real word or name.
-Alynea's title, Algid Archmage, just means 'powerful mage of cold' (so 'Ice Mage').
[Added: 16/8/19]

Appearance: A winged human-like creature, perhaps another half-harpy, with greyish-blue and silvery-white wings. Her outfit shares similar colours to her wings, but her hair is a slightly brighter blue to the rest of the blue's on her person.

Summon: Use the Broken Keystone anywhere. There are no biome, time, or depth specifications to this fight. The description of the keystone only reads 'Summons the Keystone's former Guardian'.

Fought: Post-Moon Lord
Specification: ???
Special?: No
Progression?: Yes, future bosses

Stats:
Health:
-100,000 (125,000) (Aegis)
-200 (350) (Aegis' Echo, Aegis Guard)
Attack:
-80 (150) (Aegis Melee, Echo Melee)
-75 (138) (Sword Beam, Guard Melee)
-70 (130) (Geode Trail, Energy Eruptor)
Defence:
-50 +80% Damage Resistance (Aegis)
-999 +99.9% Damage Resistance (Echo, Guard)

[All of Aegis' attacks have a 20 (33)% chance to inflict Unstable Energy]

Attacks:
Phase 1:
Aegis flies around near the player, usually keeping a fair distance but not often getting far enough away to go off-screen. She attacks every few seconds, usually summoning a lance to attack with.
-She may charge at the player, covering a long distance at a high speed
-She may charge at the player horizontally, perform a short loop, and then lunge downward at the player again afterwards, usually all in 1 swift motion
-She may thrust the lance forwards, summoning 4 sword beams either in an arc or fired 1 at a time in rapid succession.
At 50 (60)% health she will transition into her 2nd phase.
Phase 2:
Summons two 'Aegis' Echo' enemies, as well as 10 (12) Aegis Guards around herself. Also gains two new attacks as well.
-The Echoes will simply line themselves up with the player and charge at them occasionally, not performing any special attacks.
-The Aegis Guards orbit Aegis continuously, increasing in speed with each one destroyed by the player.
-The first of her two new attacks has Aegis slam her lance down towards or into the ground before 3 clusters of crystal appear below the player at random positions. After about a second, long 'snakes' of crystals begin trailing upwards from the crystal clusters. After a few more seconds the clusters and the 'snakes' disappear.
-The other new attack has her point her lance to the sky and then swing it downward, launching 5-7 'Energy Eruptor' projectiles. These slow down until they stop and then hang in the air for ages. If the player gets too close to them they will trigger, where after only about half a second they will explode. Both the projectile and explosion deal the same damage, but the explosion's hitbox is larger.

Defeat: Flinches, dispels the Guards and Echoes, and then flies upwards and disappears. As expected, all of her minions disappear at the end of the fight.

Drops: A lance for melee, tome for mage, bow for ranger, and some wings. Also drops the Guard's fragment, used to make a more defensively-oriented summoner armour. Drops 2 accessories, the first providing 2 orbiting Aegis Guards around the player and the second, the Arcane Aegis, providing a few defensive buffs. In Expert Mode she also drops the Gilded Essence; used to upgrade a few accessories like the Ankh Shield and Paladin's Shield.

Hints:
-Fast-firing weapons can be useful for dealing with the Echoes summoned in the 2nd phase, as they almost always only take 1 damage
--Similarly, piecing and AoE weapons are good for the guards that surround Aegis as well, as more than one can be hit at a time
---However, Damage-over-time effects prove effective on the boss and all of its minions, even if they are immune to the majority of them
-Despite having only few attacks, her attacking speed is fast, her fighting style aggressive, and her debuff devastating.

Extra Info:
-Aegis was designed back in the earlier parts of Boss Ideas 5, though never actually received a sprite due to characters like Crimson Aether and Skye either being made or brought back as well. Therefore, she finally gets a chance to shine now.
-I animated wings at last without them looking weird *cough* Ember Beast *cough*
-Despite the fact Aegis seems to actually be the least protected, having her hands, face, waist, and legs exposed, she actually has some of the highest defences and resistances of any boss designed to be beaten directly by the player.
-The Keystone used to hold some sort of entity, but currently it is unknown what it actually withheld.
[Added: 16/8/19]

Appearance: Three powerful warriors who have been brought over from another reality, perhaps to seek sanctuary, or perhaps to fight. The three, seen previous in Boss Ideas 4 & 5, are Storm, Magia, and Charlotte; though their designs have been updated and modified, with a lot of blacks and greys added to their old designs.

Summon: Use the Temporal Rift Key at the surface. Upon use, it will begin raining and the sky will darken to a grey, with occasional flashes to simulate lightning. All 3 appear at the same time, and they do not despawn unless the player dies or exits the world; chasing the player across the whole map if they try to escape.

Fought: Chaos Mode
Specification: ???
Special?: Unlisted Boss, Uncraftable Summon, 'Crossover/Callback Boss'
Progression?: No

Stats:
[Storm]
Health: 6,500,000 (9,000,000)
Attack:
-500 (770) (Melee, Spear)
-520 (803) (Lightning, Static Orb, Static Orb Bolt)
-480 (746) (Bolt)
-600 (900) (Overload, Zeus' Bolt)
Defence: 100 +10% Damage Reduction

[All of Storm's attacks inflict Electrified]

[Magia]
Health: 6,000,000 (8,560,000)
Attack:
-510 (784) (Melee, Sword)
-480 (746) (Fireball, Lava Globules)
-520 (803) (Lava Ball, Flame Twisters)
-600 (900) (Eruption)
Defence: 140 +12% Damage Reduction

[All of Magia's attacks inflict On Fire! and Crimson Ember]

[Charlotte]
Health: 5,650,000 (8,000,000)
Attack:
-480 (748) (Melee, Sword)
-470 (735) (Spike Traps, Fine Thread)
-20 (30) (Rapid Stab) (Ignores Defence, Damage Reduction and Invulnerability Frames)
-570 (850) (Thread Lashes)
Defence: 100 +6% Damage Reduction

[All of Charlotte's attacks inflict Hypersensitivity]

Attacks:
All 3 of them follow the player, but only 2 will attack at a time. Each one has their own unique attacks to use, though certain ones have restrictions as to when they can be used, as opposed to all of them having access to all of their attacks at any given time. Usually, long-ranged jumps and running along the ground can be used to follow the player, but if the player is too far away when one goes to attack, they may use teleportation to reach the player quickly - however some attacks will have them teleport regardless of distance to reach their positions.

[Storm]
-Storm may simply jump/appear above the player and bring her spear down at high speed. While the spear's hitbox may seem slim, it is accompanied by a large bolt of electricity as it and Storm descend
-Storm may hold the spear upwards, summoning 4-5 (5-7) lightning bolts to fall from the sky randomly nearby. These bolts linger for a brief moment before fading.
-Storm may summon a 'static orb' which acts like the Lunatic Cultist's lightning orb, though it moves slowly towards the player as it launches its lightning bolts, and the orb itself is also able to damage the player
-She may summon small lightning bolts from off-screen to travel horizontally and vertically across the arena. The bolts are fairly slow, however. This is only used below 75% health
-She may release small lightning bolts from her spear's tip outwards in all directions randomly. Up to 20 (35) can be launched at a time, with the bolts accelerating as they travel. This is only used below 75% health
-Storm's first 'special' move is the Zeus' Bolt, where she throws the spear towards some nearby ground. After doing this a large bolt of lightning will strike the spear, followed by 2 bolts either side, then 2 more follow in succession roughly 8 blocks out from the other bolts, then 2 more; continuing until the 5th wave of follow-up Zeus Bolts are summoned. This attack cannot be used alongside other 'special' moves (Unless below 30% health)
-Storm's other 'special' move is rather simple, with her simply releasing a massive explosion of electricity around herself, with a radius of about 10 (12) blocks. It also launches off 4 more large lightning bolts in a + shape. Like other special moves, it cannot be used alongside other special moves (Unless below 30% health)

[Magia]
-Magia may swing her sword in the player's general direction, summoning between 10-14 (12-16) fireballs which arc through the air, affected by gravity. They explode on contact with solid objects.
-Magia may summon a large lava bubble that slowly moves in the player's direction for a couple of seconds before stopping. Once it stops Magia will appear next to it and strike it with her blade, causing it to explode. The smaller globules of lava are launched outwards in random directions for a few seconds before the bubble disappears completely.
-Magia may summon up to 3 'lava twisters'. These appear at the player's position as small, harmless orange particles before erupting into large tornadoes of fire that hang in the positions they were summoned in (launching a few fireballs that arc through the air). They persist for a few seconds before disappearing again. This attack is only used under 75% health.
-Magia's 'special' move is the Eruption. She coats herself in fire, launches numerous flaming rocks and herself into the sky, before the flaming meteors begin falling from the sky. The last meteor to fall actually contains Magia, which is also the only meteor guaranteed to fall directly above the player. Upon hitting solid blocks they will explode violently, though Magia will emerge and slam her sword downwards when emerging from her 'meteor'. Cannot be used alongside other 'special' moves (Unless below 30% health).

[Charlotte]
-May appear near the player, hanging in the air, before dashing quickly at the player, falling down to the ground again at the end of the dash.
-She may throw numerous projectiles similar to spiked balls. 3 different types are thrown. The first is a simple one that is affected by gravity and lingers on the ground, the second is affected by gravity and bounces until it despawns, and the third isn't affected by gravity and can stick to both walls and ceilings as well as floors.
-She may disappear, creating up to 3 'fine threads'. Fine Threads appear to be part of a string or wire; being only thin silvery 'projectiles' that hang in-place once summoned and linger for a while, dealing damage on contact.
-She may appear near the player and rapidly thrust her sword forwards repeatedly, roughly around 15-20 times within as little as 1.5 seconds. Each individual stab does its own damage; ignoring both invulnerability frames and defences, allowing the attack to rapidly rack up damage. This attack can only be used under 75% health
-Her 'special' attack has her summon 3-4 more 'fine threads' randomly nearby, and to take another and violently lash it around herself, creating an AoE of damage around herself (akin to Hornet's Gossamer Storm). Special attacks cannot be used alongside other special attacks (Unless under 30% health)

Defeating either Magia or Charlotte will remove them from the fight completely. Defeating Storm first will remove her temporarily, but if only Magia or Charlotte is left active (so when 2 are defeated) Storm will assist whoever is left, unable to take damage.

Defeat: The three of them appear near to each other, facing the player. They bow (or something like bowing), before disappearing one at a time, with Storm staying slightly longer than the rest.
-All 3 of them can move in as NPCs, able to sell weapons, accessories or items based on ones found in Boss Ideas 4 & 5 (or, in some cases, the same items).

Drops:
Each will drop an item as they leave, regardless of the order they are defeated. If in Expert Mode, Storm will drop the Treasure Bag.
-Charlotte's drops include a rapid-use sword, an infinite-use upgrade to the Spiky Ball, throwing knives, and a summoning staff that summons spiders that can also leave damaging threads occasionally. She may also drop the Silk String which increases arrow speed and damage.
-Magia's drops include a sword that launches exploding fireballs, a staff that summons fire tornadoes, a rapid-fire upgrade to the Flower of Fire, and a more powerful Flamelash. She also drops the Magma Shield which removes the damage taken from On Fire! and boosts the player's damage when inflicted with a fire-like debuff.
-Storm's drops include a spear that can count as either melee or throwing, a tome that drops lightning, a magic staff that works like the magnet sphere, and an accessory that boosts movement and flight speed, grants a dash, and also increases flight time. In Expert Mode she also drops a Abandoned Realm Compass, which may perform a few random functions upon taking damage.

Hints:
-As Charlotte has the lowest health, defence, and the ability to inflict the most devastating debuff of the three, Charlotte should probably be dealt with first
-As Storm will return if only Magia or Charlotte hasn't been defeated yet, fighting Storm last means that the player only has to deal with 1 boss at the end instead of 2.
-The special attack's handicap is that only one of the three can use it at a time, prioritised in order of highest health goes first. Therefore, the Expert-Mode softening of this handicap means that, as soon as one gets to 30% health, the player may have to begin dealing with 2 special attacks at a time, as the higher-health ally will be prioritised (so they get to use it), but they also can because of the additional Expert-only rule.
-As with most Chaos Mode Bosses, Human Bosses, and Multi-Boss Bosses; dashes and teleports via the Rod of Discord are VERY useful.
-Trying to teleport away from the boss will simply cause them to teleport as well, so the player cannot cheat and remove individuals from the fight.

Extra Info:
-The idea of having alternate universe versions of characters was an idea originally intended to be used in Boss Ideas 5, though it was scrapped later in favour of other bosses.
--The universe they seem to have come from, judging by their black cloaks and unusual unities, is likely a bad one.
-Originally, characters like Garagon and Skye were going to be used instead. However, due to animation difficulties with Garagon and Skye, Charlotte actually ended up receiving the placement of 3rd member instead. (Plus Garagon alone is probably stronger than the others, so the fight wouldn't seem too well balanced.
-Magia and Storm are actually weaker here than in their own respective boss fights, when considering health.
--However, due to the fixes to Chaos Mode, their damage has been drastically improved upon.
-Storm's outfit beneath her cloak appears to be the same one used in the Lightstorm fight back in Boss Ideas 5 (#100, to be exact).
--Storm's spear is also very similar, but now features a predominantly black design as opposed to its original silvery white.
-Magia's design, outside of the added half-cloak over her shoulder, remains mostly the same.
--However, he sword has been given a different design; lacking the free-flowing red cloth, gaining two more spikes on the blade itself, and having a more jagged design for the hilt.
-Charlotte's design and weapon are almost identical, outside of her cloak.
-Most of Storm's attacks are ones taken from some of her previous fights, some directly taken over and others adjusted to make them more or less difficult.
--Charlotte's attacks, for the most part, are also taken from her fights. However, the rapid-stab attack was taken over from one of Storm's fights as opposed to Charlotte's own
---Magia's attacks were completely new for this fight, as opposed to being brought over
[Added: 17/8/19]

Well, thank you for taking a look at the latest* boss ideas page. It ended up being put into production earlier than expected, but at the same time took longer because the sprites (at least most of them) are
A N I M A T E D

[*Until the next one, obviously :P]

Anyways, you may have also noticed than unlike #5, they are separated into 15s and not 20s. This is mainly for a quality control of sorts, as less space means more competition for placements. Animated sprites also adds to the control as bosses with little effort don't exactly make for good animations.

You may have also noticed that, while separate from Boss Ideas 4 & 5, it shares some features like the setting (as Chaos' influence still exists due to characters like Terminus) and Chaos Mode still being around as a whole. Some is intentional for the lore, other parts are just for comparisons to other bosses and enemies or to separate the tough bosses from the REALLY tough ones.


[This page wins the Trending Tomato award for managing to reach #1 in the Popular Guides section]

Anyways, I hope you enjoyed, and see you all next time :)

Oh! One more thing!

https://discord.gg/URhX8GU

There's a Discord now!
I mean, it's still kinda WIP, but it should still be alright to use if you want to post ideas or chat or whatever

Anyways, see ya!