So, there were a few reasons as to why these weren't just placed in Boss Ideas 5, being that the theme was different, the number neat, and the layout also altered. Due to this 'Project Arterion' has been made its own separate thing and is less likely to be expanded upon too late into the future.

Another few things to note:

1. The themeing of these bosses is different to my usual stuff, with a lot of them being more, I suppose, 'edgy' than my usual boss designs as of recent. After doing this I'll be returning to my 'regularly scheduled designs' as it were.

2. These bosses do NOT fit into the lore of the vanilla game nor the lore built up across both Boss Ideas 4 and 5. It is its own separate thing.

3. Fury Mode, a new difficulty, has been added. It adds changes to all bosses from this page and in the vanilla game, making them all significantly harder.

4. The layout changes are done to adjust to the differences between these bosses and the previous ones, so I shall describe the new layout below:
Name: [Picture of Boss] + Brief description of the boss
Appearance: What the boss looks like, may reference other enemies and bosses if needed.
Summon: The method for summoning the boss, usually via a usable item.
Fought: Both the section of the game and also which bosses are fought prior and after it
Stats: Boss' statistics, with any significant minions having their own separate boxes to avoid clutter
Attacks: The boss' actions when it has been summoned, sometimes separated into phases for more complex bosses
Defeat: The boss' defeat animation, whether they explode into gibs, puff into smoke, or simply retreat.
Drops: Examples of what sort of items the boss would drop in-game, as well as Expert and Fury mode-exclusive drops.
Hints: If they were to be fought what precautions could be taken or what you should watch out for
Extra Info: Any additional information about the boss or its drops. Most of the time it is just additional trivia and not too important

- The 'first' boss of Arterion and also the one who must be defeated in order to activate Fury Mode.

Appearance: A large red slime with large black tendrils wrapped around its body. What appears to be a skeletal face can be seen peering out from its centre.

Summon: Use the Fury Core anywhere in the world at night. The boss can be fought in any biome and at any depth, but will despawn by falling through the ground once it reaches daytime.

Fought: Pre-Hardmode
The Fury -> King Slime
Special: Used to activate Fury Mode if it's the first boss defeated in an Expert World
Compulsory?: No, except for Fury Mode.

Stats:
[The Fury]
Health: 1,500 (2,750) (4,000)
Attack:
-35 (60) (100) (Melee)
-(40) (60) (Shockwave)
Defence: 5

[Fury Slime]
Health: 30 (60) (100)
Attack: 30 (50) (78)
Defence: 5

Attacks:
[Normal]
The Fury will hop towards the player in a manner similar to King Slime. It will perform 2 smaller jumps before performing its 3rd, higher jump. Every 100 damage it takes will cause it to spawn a Fury Slime, which act like regular slime enemies despite their large size.
[Expert]
Gains two abilities in Expert Mode, first is the ability to teleport to the player if they are too far from The Fury or are deemed unreachable, and also it can perform a 'super jump' where it jumps straight upwards and slams down into the ground, creating a shockwave that travels 5 blocks in each direction.
[Fury]
Speeds up its movements. Shockwaves now spawn 2 Fury Slimes upon landing and the shockwaves travel outwards by 10 blocks. Uses the super-jump more frequently.

Defeat: Explodes into slimy gibs, leaving behind an additional 3 Fury Slimes. Fury Slimes do not despawn upon the boss' defeat and remain aggressive towards the player until defeated or despawned.
-If it is the first boss defeated on an Expert world it will activate Fury Mode permanently for that world

Drops: Drops Fury Gel, a material used to craft an early-game armour set and some basic weapons for the main classes like a sword, magic staff, bow and summoner staff.
In Expert Mode it also drops both the Fury Fragment; a material for a late-game item, and the Slimed Core which grants increased movement speed but all blocks become slippery.
In Fury Mode it also drops the Backtracker's Brooch which mildly boosts damage against any bosses defeated previously.

Hints:
-As this fight bears similarities to King Slime some may think that strategies like the 'rope ladder' technique will work. They do not in Expert or Fury Mode because of its super-jump attack.
-In Normal Mode the player can seal themselves in a box and have minions kill the boss, but as it gains teleportation abilities in Expert and Fury mode it doesn't work on those difficulties.
-Moving away from the boss constantly using things like hermes boots can allow the player to keep a distance throughout the whole fight, which makes it much easier
--Don't move too far away, however, as it may teleport to you and potentially teleport directly on top of you
-Platforms also help as they can catch its super-jumps and some of the slimes spawned.

Extra Info:
-The requirement to defeat this boss as the very first boss in order to successfully activate Fury Mode isn't mentioned in-game until the player befriends Callie later-on, which means that if the player doesn't simply look it up then Fury Mode is made to be more for a 2nd playthrough where the player has encountered most of the bosses prior.
--The Fury MUST be defeated as the first boss, or Fury Mode will not activate.
-As Fury Slimes are spawned at specific amounts of health then the player can actually track the damage needed to defeat the boss including all of the slimes (excluding the post-defeat slimes). This is 1950 in Normal, 4370 in Expert and 8000 in Fury Mode.
-Despite quite an intimidating appearance for a slime it is actually the weakest boss in the game statistically; even weaker than King Slime
[Added: 27/7/19]

(To ensure every boss is counted in-order the Fury Mode changes to the vanilla bosses will be placed in-between others to show progression).

King Slime:
Health: (3,500)
Attack: (100)
Defence: 10

Attacks:
[Fury]
-Can now perform an additional long-jump which covers a large amount of horizontal ground
-Launches spikes upwards at players who are above him that are affected by gravity. The longer the player is above him the higher the spikes will travel before they begin their descent.

Drops: Fury Mode drop is the Slime Surfer, which when equipped allows the player to move faster on Slime Blocks and Frozen Slime Blocks, and bounce higher on Pink Slime Blocks.

Eye of Cthulhu:
Health: 4,800
--3,000 (Phase 2 Activates)
Attack:
-35 (Phase 1)
-50-100 (Phase 2)
Defence:
-15 (Phase 1)
-5 (Phase 2)

Attacks:
[Fury]
-Charges in both phases are faster, chain dashes happen faster, and changes to Phase 2 earlier
-Phase 2's damage scales as the boss' health depletes, going from 50 at 3000 health up to a maximum of 100 once at only 100

Drops: Fury Mode drop is a throwing weapon that throws a mini Eye of Cthulhu at enemies which latches onto them for additional damage.

Desert Defenders are the second Arterion boss and is built more as an event with waves of enemies than just a single larger one. Their small size is offset by their large number.

Appearance: Numerous skeletal creatures, each covered in sandy rags. They appear lizard-like, though appear to be lacking any sort of arms or additional legs.

Fought: Pre-Hardmode
Eye of Cthulhu -> Desert Defenders -> Eater of Worlds
Special: No
Compulsory?: No

Stats:
Health: 150 x30 (250 x38) (400 x50)
Attack:
-30 (50) (75) (Melee) (Inflicts Bleeding)
-27 (45) (70) (Sand Ball)
-(50) (83) (Sand Slash) (Inflicts Bleeding*)
Defence: 20

[*Only inflicts Bleeding in Fury Mode]

Attacks:
[Normal]
Spawn in so that there are always 4 active, with new ones replacing each one defeated. They slowly walk towards the player at first. They aren't immune to knockback, but are fairly resistant.
When 20 have been defeated the final 10 will gain the ability to occasionally throw balls of sand at the player that are affected by gravity.
[Expert]
They spawn so 5 are active at a time and have increased movement speed. When 5 have been defeated they gain the sand ball attack. They have a 75% knockback resistance.
Once 20 have been defeated they gain a new ability where they stand still briefly and then fire a small crescent-shaped projectile at the player that passes through tiles. This attack becomes more common again once 30 have been defeated.
[Fury]
6 are active at a time and they both move faster and have 85% knockback resistance, meaning that they barely flinch at most attacks now. The sand ball attack can happen at any time and the crescent projectiles are launched once 10 have been defeated. Crescent attacks can also inflict Bleeding.

Defeat: Explode into gibs of bone and fabric. Boss is considered 'won' when all of the Desert Defenders have been defeated.

Drops: Individuals only drop money and Tattered Cloth, while the last one to be defeated drops the loot. A throwable bottle of sand, a spear or a tome.
Expert Mode allows it to drop both the Second Chance; which revives the player with 20 health after death with a 4-minute cooldown, and the Guardian Fragment; used to craft a late-game item.
In Fury mode it drops the Defender Staff which summons Desert Defenders to assist the player.
[In Expert or Fury Mode they will also drop the Returning Fragment after Bone Meal has been defeated]

Hints:
-The player might benefit from making an arena with many different platforms to avoid being trapped by the Defenders
-The player can beat the Normal Mode fight by sealing themselves in a box as they don't have any attacks that can pierce blocks until Expert Mode
-High-knockback weapons can do a fair job of holding them back, but knockback becomes less effective at higher difficulties.
Extra Info:
-The Desert Defenders were designed a long time before this page and thus their sprite is underwhelming in comparison with others, meaning it may be changed in the future.
-They receive a buff once Bone Meal is defeated, though don't actually gain any new drops outside of additional money and one material
-In Normal Mode they have a total of 4500 health, in Expert they have 9500, and in Fury Mode they have 20000 in total.
[Added: 27/7/19]

(Covers Eater of Worlds, Brain of Cthulhu, Queen Bee, Skeletron, and the Wall of Flesh. There is a large gap as the next boss is early Hardmode)

Eater of Worlds:
Health: (140) / (300) / (385)
Attack: (70) / (30) / (28)
Defence: (10) / (12) / (15)

[Order: Head, Body, Tail]

Attacks:
[Fury]
-Can launch Cursed Fireballs occasionally in-place of Vile Spit. Cursed Fireballs replace Vile Spit more often the fewer segments remain.

Drops: In Fury Mode it drops the Vile Spear, which acts like a weaker version of the Scourge of the Corruptor.

Brain of Cthulhu:
Health:
-(2000) (Brain)
-(275) (Creeper)
-(9999) (Immortal Creeper)
Attack:
-(70) (Brain)
-(50) (Creeper)
Defence:
-14 (Brain)
-10 (Creeper)
-(50) (Immortal Creeper)

Attacks:
[Fury]
-Once into Phase 2 it spawns an Immortal Creeper. This acts like a regular Creeper but isn't intended to be defeated. While it is close to the Brain it will become immune to damage but will slow down, though once the Creeper charged at the player or the Brain teleported the Brain would go back to its regular movement speed.

Drops: In Fury Mode it drops the Leeching Knife which throws a single life-stealing dagger on use.

Queen Bee:
Health: (6,850)
Attack:
-(60-80) (Melee) (Inflicts Venom)
-(50-75) (Stinger)
Defence: (8-20)

Attacks:
[Fury]
-Damage also increases as its health decreases. Contact damage from its body may inflict Venom as opposed to Poison.

Drops: In Fury Mode it drops Royal Honey which makes Bees and Hornets friendly and drenches the player in Honey whenever they take damage.

Skeletron:
Health:
-(10,000) (Head)
-(2,000) (Hands)
Attack:
-(85) (Head)
-(60) (Hands)
Defence:
-(12) (Head)
--(250) (Hands not Defeated)
-(16) (Hands)

Attacks:
[Fury]
-Hands, at under 10% health, can also fire skulls at the player and, upon death, release 4 skulls outwards in a '+' shape.
-Skeletron can also launch skulls whilst spinning, though with significantly decreased frequency

Drops:
In Expert Mode it also drops the Dungeon Fragment; a material for a late-game item
In Fury Mode it drops the Marrowmere, a sword that launches homing skulls that can be upgraded later.

Wall of Flesh:
Health: (15,000)
Attack: (500)
Defence: (20)

Attacks:
[Fury]
-Granted enough contact damage to almost certainly instantly kill the player on-hit, forcing the player to keep their distance. Fires lasers more often and Hungries spawn more frequently whilst at lower health.

Drops: In Fury Mode it will drop the Core Heart which, upon consumption, boosts the player's health slightly and increases Accessory slots by 1 (For a maximum of 7 alongside the Demon Heart from Expert Mode).

A trio of skeletal serpents. They are always on the hunt for food, despite not really having a body left to feed nor a stomach to actually digest anything.

Appearance: Three serpents made of bones, with their bodies seemingly made of ribs and a long spine running down from their heads. Ones' head appears human, the second one's head a mouth and the third appears more alien; with numerous eyes and a strange indent where its jaw would be.

Summon: Use the Bone Dust at night. The boss can be taken to any biome or depth without any problems, though will still despawn once it becomes daytime.

Fought: Hardmode
Wall of Flesh -> Bone Meal -> Mechanicals
Special: No
Progression?: No

Stats:
[Worm 1]
Health: 10,000 (14,850) (22,000)
Attack:
-60 (100) (155) (Head Melee)
-30 (50) (90) (Body Melee/Bones)
Defence: 40 (40) (50)

[Worm 2]
Health: 10,000 (14,850) (22,000)
Attack:
-75 (128) (180) (Head Melee)
-40 (65) (110) (Body Melee/Bones)
Defence: 30 (30) (36)

[Worm 3]
Health: 13,000 (18,000) (30,000)
Attack:
-60 (100) (155) (Head Melee)
-30 (50) (90) (Body Melee/Bones)
Defence: 30 (30) (36)

Attacks:
[Normal]
-The three worms burrow through the ground, gradually gaining speed as its health decreases. Occasionally, when under half health, they may launch 3-4 bone projectiles which are affected by gravity and shatter upon contact with solid blocks.
[Expert]
-The additional speed granted by losing health is increased significantly. 4-5 bones are launched at a time and have a wider range. Bones are used at 80% health and can pass through blocks until reaching the same vertical height as the player.
[Fury]
-Additional speed gained by losing health is even greater. For each one defeated the others gain 10% damage reduction added additively (10% when 2 are left, 20% when only one). Bones are launched with increasing frequency at lower health and are usable at any amount of health.

Defeat: They explode into skeletal gibs upon defeat, as expected.
-Upon their defeat the Desert Defenders will receive a buff to their stats, with 400 (550) (800) health, 50 (75) (100) melee damage, 45 (70) (90) sand ball damage and (90) (120) damage on their Expert 'sand slash' attack. They also gain an additional drop.

Drops:
-In Normal Mode they will drop Archaic Bone; a material used for an armour set and a few different weapon types. Can also drop a few miscellaneous items like the Beam Sword, Vitamins, Nazar, Armour Polish, and large quantities of Bone and the Bone Wand.
-In Expert Mode they can also drop both the Skeletal Fragment; a material for a late-game item, and also the Duster which consumes Bone to launch large volleys of damaging dust.
-In Fury Mode it also drops a flail shaped like one of the Bone Meal worms that works like the Solar Eruption, though only inflicts Bleeding.

Hints:
-As they are worm bosses piercing weapons can deal significant portions of damage not just to one of them, but potentially to all 3 at the same time if they can be lined-up.
--AoE weapons can also deal large damage to all of them at the same time, like Grenades.
-The player may want to stay on a platform far from the ground to avoid their melee damage, but will still have to be wary of their bone-launching attacks and increased movement speed potentially getting them up to the platform.
Extra Info:
-Despite having more health than a few of the later bosses, the fact that it is divided amongst 3 worm bosses means that it is significantly easier to get through
-Bone Meal's name being a pun was actually a significant part of the designing process for this boss
-Originally, all 3 worms making up Bone Meal were going to be identical. However, a re-design occurred mid-way through designing them that had them all have different heads and, along with that, different stats as well
-Bone Meal was the last boss to actually be planned for the page, turning out to mainly be an after-thought brought on by a suggestion which then became another boss to fill a large portion of the game where no new bosses were added.
[Added: 28/7/19]

(Logs the changes made to the Twins, Skeletron Prime, the Destroyer and Plantera)

Twins:
Health: (42,500) / (46,750)
--(28,000) / (30,000) (Phase 2 Transition)
Attack:
-(100) (Retanazer: Phase 1 Melee, Phase 2 Rapid Fire)
-(112) (Retanazer: Phase 1 Laser / Spazmatism: Phase 1 Melee)
-(130) (Retanazer: Phase 2 Melee / Spazmatism: Phase 1 Fireball, Phase 2 Fire)
-(145) (Retanaser: Phase 2 Laser / Spazmatism: Phase 2 Melee)
Defence:
-(30) (Phase 1)
-(40) (Phase 2)

Attacks:
[Fury]
-Phase 1's lasers and fireballs are launched with increasing frequency the closer they get to Phase 2.
-Spazmatism can still launch fireballs once in Phase 2, though with decreased frequency.
-Retanazer is able to inflict On Fire! with its laser attacks, and lasers are fired faster in Phase 2.

Drops: In Fury Mode they also drop the Flamespitter; a flamethrower-like weapon that launches explosive Cursed Fireballs at the cost of gel.

Skeletron Prime:
Health:
-(60,000) (Prime)
-(1x) (Limbs)
Attack:
-(100) (Melee/ Sawblade)
--(200) (Spinning)
-(80) (Laser/ Missile)
-(1.2x) (All Limbs)
Defence:
-(30) (Prime)
--(60) (Spinning)
-(1x) (Limbs)

Attacks:
[Fury]
-All Limbs act the same and mostly have the same stats outside of a 1.2x multiplier on damage and bombs no longer ignoring platforms. Prime not only has better stats but also gains abilities based on what limbs have been destroyed.
--If the Laser is destroyed then it can fire lasers in a '+' or 'x' shape roughly every second when not spinning
--If the Vice is destroyed then its headspin attack has increased movement speed
--If the Saw is destroyed then Prime can launch sawblades at the player whilst spinning
--If the Cannon is destroyed then Prime may occasionally fire missiles directly at the player every few seconds

Drops:
-In Expert it additionally drops the Mechanical Fragment; a material for a late-game item.
-In Fury Mode it drops the Mechanical Extremity Pack; which gives the player 4 miniature versions of Prime's limbs attached to the player which attack nearby enemies.

Destroyer:
Health: (200,000)
Attack:
-(400) (Head Melee)
-(80) (Body Melee)
-(68) (Tail Melee, Laser)
Defence: (40)

Attacks:
[Fury]
-The Destroyer's head can now nearly instantly kill the player on contact and has increased defence. The Destroyer's lasers now inflict On Fire!. When under 30% health Probes can spawn in from off-screen every few seconds. Probes also have a reduced chance to drop hearts, which is reduced even further for ones spawned under 30% health.

Drops: In Fury Mode it may also drop the Destroyer; a throwable sawblade-like boomerang that has high damage, speed and piercing abilities.

Plantera:
Health: (62,000)
Attack:
-(140) (Phase 1 Melee, Thorn Ball)
--(185) (Phase 2 Melee)
-(118) (Poison Seed)
-(100) (Seed)
Defence: (20)

Attacks:
[Fury]
-Plantera fires Poison Seeds more commonly, and they can actually inflict the Venom debuff on hit. Plantera's attacks all inflict Poisoned. Plantera can still drop thorn balls in Phase 2, but does so less often. Phase 2's movement speed is increased slightly.

Drops: In Fury Mode Plantera drops the Shredder; a guitar-like magic weapon that launches shockwaves which can reflect off of walls and pierce enemies.

The titular boss of the project and presumed main antagonistic force of the lands; connected in some unknown ways to the others.

Appearance: Arterion appears, to an extent, like a skull. It has two sunken, almost sorrowful eye sockets and a mouth entirely covered by what seems to be some strange fleshy substance. Numerous tendrils are attached from this fleshy clump to Arterion's skull and many others dangle from it aimlessly.

Summon: Use the Cursed Skull anywhere in the world at night. The boss will despawn during the day, but is unaffected by changes in depth and biome. However, there is still an additional benefit if fought in the World Evil and another in the Underworld if fought in Fury Mode.

Fought: Hardmode
Plantera -> Arterion -> Golem
Special?: No
Progression?: Yes* (Post-Game)

Stats:
Health: 45,000 (70,000) (88,500)
Attack:
-60 (100) (165) (Melee)
-50 (85) (130) (Blood)
-55 (92) (143) (Red Skull)
-48 (82) (125) (Red Seeker)
Defence: 15

[All of this boss' attacks inflict Bleeding]
[In Expert/Fury Mode, this boss' 'Melee' and 'Blood' attacks can inflict Red Fury]
Debuff: Red Fury
"Negative Emotion Overwhelms You"
-Player loses 30 defence, takes 10 damage per second, and gains +5% damage

Attacks:
[Normal]
Phase 1:
Will either be flying above the player or moving towards them, based on the upcoming attack.
-It may rain down blood-like droplets from its body down upon the player
-It may launch numerous blood-like droplets upwards which quickly fall back down again in arcs
-It may disappear before re-appearing nearby, charging at the player before chasing them for a few seconds. A couple of seconds later and it will disappear.
Upon being taken down to 50% health it will transition into its 2nd phase. In this phase more tendrils appear and the dangling ones grow longer and squirm uncomfortably.
Phase 2:
In Phase 2 Arterion continues doing what it had in its other phases, but also gains new attacks as well.
-It may chase the player, summoning occasional volleys of 4 Red Skulls. These disperse outwards from where they are summoned in a large spiral formation, spiralling outwards until they despawn.
-It may fly overhead and summon Red Seekers; eye-like projectiles that follow the player's horizontal location while slowly descending, despawning on contact with solid blocks.
[Expert]
-Certain attacks can now inflict Red Fury, more droplets are spawned during attacks and rain is dropped continuously as opposed to just when the player is directly beneath the boss.
-Red Skulls' rotations are faster and Red Seekers do not despawn on contact with solid blocks.
[Fury]
-More droplets are spawned with its Phase 1 attacks and its movement speed when chasing the player is increased.
-Summons 5 Red Skulls per volley and Red Seekers descend slower than before and more are summoned before switching to new attack.

Defeat: Arterion stops, transitions back to its 1st phase's design, and then ascends upwards rapidly, slowly fading as it does so.

Drops:
-In Normal Mode it drops a flail, sword, bow, 2 magic tomes and a summoner staff that summons Red Seeker minions. Also drops Eldritch Clumps; materials for a few additional weapons, an armour set, and a couple of other items.
-In Expert Mode it drops the Swollen Charm, which releases damaging blood droplets upon taking damage, and also the Nightmare Fragment; used as a late-game material.
-In Fury Mode it will drop Divided Paper A normally, B in the world evil and C in the underworld. Combining them creates the Union Scroll which boosts player stats slightly when equipped.

Hints:
-Once he begins to approach half health be sure to keep your distance as, upon transitioning to his second form, the size of his hitbox will increase.
-Arterion's defence is relatively low but his health is quite high for when he is fought. Bringing high DPS weapons, even if their base damage is low, may be useful still.
-A dash ability could be useful when he begins to chase the player during or after some attacks to grant quick bursts of mobility away from Arterion
-Arterion does extremely high damage in Expert and Fury mode, so practising on Normal mode may make for good preparation.
-Many of his attacks come from above, meaning flying upwards may be risky
Extra Info:
-Arterion was the first boss designed for the project, and is now the titular boss of it as well.
-Arterion is one of the only two bosses who, upon defeat, are not killed and simply retreat.
-Up to this point Arterion is the only enemy able to inflict Red Fury
-The individual parts of the Divided Paper talk about an otherworldly entity brought to this land to bring its destruction, however due to large pieces being missing or damaged some of it apparently cannot be read
--From what is visible, it is likely Arterion himself may be the threat they are referring to, mainly because the paper pieces are his own Fury Mode drops.
[Added: 29/7/19]

(Fury Mode changes for Golem, Duke Fishron, Lunatic Cultist and the Moon Lord; the final 4 vanilla bosses. From this point forward it's only Arterion's own bosses.)

Golem:
Health: (30,000) (Head / Body)
Attack:
-(140) (Head Melee / Fireball / Fist Melee)
-(150) (Laser)
-(160) (Body Melee)
Defence: (30)

Attacks:
[Fury]
-Fists are now invincible and the body's health is increased drastically. Fireballs deal set damage based around its original highest amount of damage and move faster. Lasers and fireballs both inflict On Fire!.

Drops: In Fury Mode the Golem will drop the Guardian Spark; an accessory that boosts defences and damage reduction when the player is standing still.

Duke Fishron:
Health: (85,000)
--(50,000) (Phase 2 Trigger)
--(10,000) (Phase 3 Trigger)
Attack:
-(170) (Phase 1: Melee, Detonating Bubble)
-(150) (Phase 1: Sharknado)
-(255) (Phase 2: Melee, Cthulhunado)
Defence: (60)

Attacks:
[Fury]
-Sharknadoes and Cthulhunadoes last longer and more Sharkrons are present in each. Bubbles inflict Chilled on hit and Fishron himself also gains a 33% chance to inflict Broken Armour alongside its other 2 debuffs.

Drops:
-In Expert Mode Fishron will additionally drop the Mutant Fragment; a late-game crafting material.
-In Fury Mode Fishron will drop the Seafarer's Soul; an accessory that boosts attack, defence and significantly increases movement speed whilst the player is submerged in water.

Lunatic Cultist:
Health:
-(55,000) (Cultist)
-(2x) (Phantasm Dragon, Ancient Vision)
Attack:
-(120) (Melee, Shadow Fireball, Ancient Doom, Fireball)
-(150) (Ice Mist, Lightning Orb)
-(200) (Ancient Light)
Defence: (50)

Attacks:
[Fury]
-Ice Balls inflict Chilled and Lightning Orbs inflict Electrified. Ancient Lights are no longer destructible and must be dodged. Attacks are faster and the Cultist teleports more often.

Drops: In Fury Mode it drops the Moondial; which boosts the player's stats at night, during a solar eclipse, or during the lunar events.

Moon Lord:
Health:
-(100,000) (Head)
-(44,000) (Hands)
-(115,000) (Core)
Attack:
-(120) (Eye Socket Melee)
-(150) (Phantasmal Bolt, Phantasmal Eye)
-(230) (Phantasmal Sphere)
-(400) (Phantasmal Deathray)
Defence:
-(55) (Head, Hands)
-(80) (Core)

Attacks:
[Fury]
-Phantasmal Deathray is able to pass through blocks until reaching the same vertical height as the player, forcing the player to dodge as opposed to simply hiding. Phantasmal Eyes are spawned from the Eye Sockets after the individual eyes have been defeated. Eyes move faster and are more aggressive once the Core is exposed.

Drops: In Fury Mode it drops the Lunar Seeker; a mount that summons a True Eye of Cthulhu with infinite flight. It is faster than the UFO Mount, but takes longer to decelerate once moving and turns slightly slower.

The Sulkers are ghostly, transparent faces that are unleashed upon the Moon Lord's defeat. While they may not seem all too threatening, they appear to have some otherworldly powers at their disposal.

Appearance: Sulkers are slightly transparent, dark-blue entities whose bodies appear like elongated faces. Their mouths are stretched out and are always open and their eyes are always weeping; which is where they received their names.

Summon: Can be found in one of two ways. Either they can be encountered naturally after the Moon Lord is defeated once night arrives, with the message 'Something lingers in the dark...', or can be summoned manually using the Reaching Pendant. It can spawn on its own until defeated 3 times, where it must then be summoned manually.

Fought: Post-Moon Lord
Moon Lord -> Sulker -> Phantasmaw
Special?: No
Progression?: Yes

Stats:
Health:
-120,000 (175,000) (235,500) (Alpha Sulker)
-30,000 (50,000) (60,000) (Separate Blue Sulker)
-65,000 (100,000) (150,000) (Blue Sulker Minion)
-(130,000) (185,000) (Purple Sulker Minion)
Attack:
-85 (150) (200) (Alpha Sulker Melee)
-70 (132) (170) (Sulker Minion Melee/ Tear Bolt)
-75 (138) (183) (Ghostly Energy)
-65 (125) (160) (Spectre Fireball)
Defence: 70 (70) (85)

[The Alpha Sulker's Expert 3rd phase can inflict Red Fury]

Attacks:
[Normal]
Phase 1:
The Sulker floats towards the player, occasionally teleporting to reposition itself. Its movement speed increases as its health decreases, as well as it slowly turning progressively more purple. It has a few different attacks to use against the player.
-It can summon smaller, blue versions of itself which teleport, wait a second, charge at the player, wait for a couple of seconds, and then repeat the cycle. Only 1 can be active at a time. (Separate Blue Seekers)
-It can launch numerous Ghostly Energy projectiles which chase the player until they despawn. They move quite fast and up to 6 are spawned in succession at a time.
-It may launch a barrage of Tear Bolts at the player, which can pass through blocks. There is a large number, but they are launched rather inaccurately.
At 60,000 (120,000) (165,000) health, it will transition to its 2nd phase. It turns purple and its expression changes slightly.
Phase 2:
In its 2nd phase it will summon 8 Blue Sulker Minions orbiting roughly 8 blocks out from its body in a clockwise rotation, teleporting with the Alpha Sulker. Each of these 5 has individual health and all of them may fire Tear Bolts at the player separate from the Alpha Sulker.
-It gains 1 new attack where it releases 3 Spectre Fireballs out in random directions, which can ricochet off of walls until they despawn.
[Expert]
Fires more projectiles with its attacks and moves faster. 4 Spectre Fireballs are launched at a time and they move faster. In Phase 2 it slowly turns red as its health goes down.
At (60,000) (95,000) health it transitions into its 3rd phase, gaining an angrier expression and turning bright red.
Phase 3:
A second ring of 5 Purple Sulker Minions spawns in-between the Alpha and outer ring, rotating anti-clockwise. These minions launch Tear Bolts more frequently.
-The Alpha Seeker attacks and moves faster at lower health, but doesn't gain any new attacks.
[Fury]
-Teleports more frequently and the Alpha Seeker will attempt to ram into the player after every teleport. 3 Separate Blue Sulkers can be active at a time. Its Spectre Fireball attack now only launches 3 Fireballs at a time, but every time they hit a solid block they will split into 2.

Defeat: Begins shaking and appears to begin melting, before bursting into a puff of smoke and some purple, blue and red globules.
-The Strange Traveller may now begin appearing at the surface during nighttime.

Drops:
-In Normal Mode it drops Spectral Globs; materials used to make a range of weapons and a few armour sets. Also drops the Spectral Bolt; a magic tome that acts like a stronger water bolt, and a couple of other tomes. Can also drop a summoner staff, sword, or an accessory that summons a bubble shield around the player after dealing damage (though it has a cooldown).
-In Expert Mode it drops Sorrowful Fragments; a late-game material, as well as the Droplet Cannon; a minigun-like ranger weapon that fires droplet-shaped bullets at increasing speed the longer it is fired.
-In Fury Mode it drops the Sulkseeker; a throwing weapon that launches a disc that releases homing droplets as it travels.

Hints:
-Fight this boss out in the open, for example at the surface and with an arena without walls. This both grants additional room to dodge and also means that the Spectre Fireballs will have minimal time active, which is especially important in Fury Mode.
-Keep your distance from the boss. This is both important for phase transitions, as it summons orbitals, but also because it means you have more time to dodge its numerous projectiles.
Extra Info:
-The Alpha Sulker is only slightly larger than the others, but is distinct as the Phase 2/3 minions orbit it and it is, for the most part, a different colour from its minions.
--The only exception is that Separate Blue Seekers share its Phase 1 colour, despite being spawn-able in the 1st phase.
[Added: 29/7/19]

A powerful mage inspired by tales of one who came before her. Unlike many creatures seen before, she doesn't appear to be a malevolent entity but, instead, human.

Appearance: A red-haired human wearing a set of black mage robes and a red scarf that moves in the wind. Her appearance, to some, may look familiar.

Summon: Found as the Wandering Traveller anywhere at the surface during nighttime. Interacting with her will cause her to taunt you and then she can be challenged by pressing the 'challenge' option beneath her dialogue box. Unlike the other bosses, she cannot be summoned using items.

Fought: Post-Moon Lord
Sulker -> Callie -> Phantasmaw
Special: Uncraftable summon, 1-time fight.
Progression?: No

Stats:
Health: 170,000 (265,000) (400,000)
Attack:
-85 (150) (230) (Melee)
-90 (158) (244) (Ruby Shard)
-88 (155) (240) (Red Bolt)
-100 (174) (265) (Red Detonator)
Defence: 70 (70) (85)

[Melee and Red Detonator inflict Red Fury in Normal Mode, though all attacks can in Expert and Fury Mode]
[In Expert all attacks except Red Bolts can inflict Vulnerability, though all attacks can in Fury Mode]
Debuff: Vulnerability:
"Your defences wither away..."
-Increases damage taken by 20% and reduces defence by 20

Attacks:
[Normal]
Phase 1:
Teleports, uses an attack, and then teleports again; a simple cycle but still quite dangerous. She has a few different attacks, used progressively faster as her health depletes.
-She may summon numerous Ruby Shard projectiles. These move towards the player, but all have different paths and arcs which can be overwhelming to try and track as they move.
-She may summon numerous Red Bolt projectiles either horizontally or vertically. Many are summoned, though there is enough space to make it through the mini bullet-hells.
-She may summon large amounts of Red Bolts out from her position, both many per volley and many volleys in rapid succession. They can either be evenly-spaced and subsequent bursts rotated to give a spiral pattern, or all released randomly-spaced.
-Finally, she may summon a Red Detonator above her head which explodes. Upon it exploding, many other Red Detonators will begin appearing nearby and exploding on their own after roughly 1.5 seconds each.
At 80,000 (110,000) (180,000) health she transitions into her 2nd phase.
Phase 2:
Callie will summon forth an entirely-red holographic clone of herself that is immune to damage and fights alongside her. Her attack speed slows, but now Callie and her clone take it in turns to attack, often having one teleport while the other attacks. This phase's attack speed also increases gradually as her health lowers.
[Expert]
-Teleports more frequently and uses more projectiles for all of her attacks. Red Bolt-oriented attacks feature more projectiles and tighter gaps between projectiles. An additional 3rd wave of Red Detonators spawn after the 2nd, though less of them spawn. Red Detonators detonate only after 1.2 seconds.
-When transitioning to her 2nd phase less attacking speed is lost in the process, meaning that Phase 2 is faster from the get-go.
[Fury]
-Additional Red Detonators are spawned in both the 2nd and 3rd waves, all projectile attacks feature more projectiles and less gaps in-between them. Fight gains an additional 3rd Phase at (60,000) health.
Phase 3:
-A 3rd Holographic version of Callie is spawned. Now 2 of them will be attacking at a time while 1 teleports, making dodging significantly harder due to the overlapping attacks and increased attack frequency.

Defeat: Callie's holographic images disappear and she falls before teleporting away moments later.
-She will now move in as an NPC, selling a wide range of items.
-If defeated prior to Phantasmaw the 'Watcher' will begin to appear in the background at night, following the player quietly from the background.

Drops:
-In Normal she can drop a spear, sword, 2 tomes, a summoning staff, a mage staff, a gun, a bow, and, rarely, a pet.
-In Expert she can drop an accessory that will revive the player to 20% health after dying, though with a 3-minute cooldown. Rarely, she may also drop a mount.
-In Fury Mode she may drop another summoning staff and another different pet.

Hints:
-As her attacks are mostly bullet-hell-themed, a lot of time may be spent dodging attacks. Therefore, homing projectiles may be good as then you only have to focus on dodging and not aiming as well.
--A Rod of Discord can also be useful when trying to dodge as entire attacks can be skipped if used right.
-The fight continuously speeds up as it progresses, so the player may benefit from refraining from attacking the boss for a while, to give time to get used to her attacks.
-This boss doesn't have a time limit as she doesn't despawn during the day. Don't try to rush the boss.
Extra Info:
-Callie is a reference to the humanoid design for Calamitas as seen in DM Dokuro's video featuring the music for the Supreme Calamitas boss. However, despite this, the brief description given for the boss suggests that it isn't Calamitas and is, instead, simply inspired by tales of Calamitas.
--Therefore, if looking from a time-based perspective, Arterion would have to likely take place somewhere after the events of Calamity.
--The other nod to Calamity comes from both of her Fury Mode items. The pet appears to be a miniature version of the Calamitas Doppelganger, and the summoning staff summons mini versions of Catastrophe and Cataclysm which attack with fireballs and a flamethrower, respectively.
-The Normal-mode pet appears to be a chibified version of Callie, and the rare Expert-mode mount is actually just one of the Callie Holograms comically piggy-backing the player as opposed to anything serious.
-Callie is the only other boss in Arterion to not be destroyed upon defeat, with the other being Arterion's 1st fight.
-Callie is the 1st boss to be able to inflict the Vulnerability debuff on the player, as well as the 1st to potentially summon the Watcher in the background upon her defeat.
[Added: 29/7/19]

Upon her defeat, Callie can move in as an NPC and begin selling items. Many of her items are either some of her drops during her original boss fight, or new ones that can only be brought from her.

Stats:
Health: 50,000
Attack: 100 (Attacks by launching Red Bolts rapidly at nearby enemies)
Defence: 99,999

Extra: While a boss is active, Callie will provide a defensive shield for all NPCs which significantly reduce the damage they take and makes Callie herself immortal. However, while the shield is up, Callie cannot attack either.
Extra: Callie regenerates health at roughly 5 health per second, which when combined with her defence makes her almost immortal.

Items:
-Her Normal-mode drops, excluding her pet (Always)
-Expert-mode drop (Expert Mode Only)
-Chibi Callie pet (Full Moon)
-Holographic mount (New Moon)
-Class-specific Medallions (Always)
-Overwhelming Wrath/Rage Potion (Always)
-Glass Cannon Potion (Night)

Descriptions:
-Normal items, Expert items, pet, mount: Same as the items she drops during her boss fight
-Class-specific Medallions: +20% Damage to certain damage types. Can be combined with certain items like the Fire Gauntlet.
-Overwhelming Wrath/Rage: +25% Crit Chance/Damage, but deals 5 damage per second to the player while the player is under its effects. Can be cancelled like most other buffs.
-Glass Cannon Potion: All attacks deal double damage to the player, but the player deals 40% extra damage

One of the final bosses of Arterion. Phantasmaw is the last thing standing between the player and their final fight against Arterion's true form.

Appearance: A large black and purple entity with a jagged crown of purple crystalised 'teeth' lining a large shadowflame. Three shadowflame eyes are below the mouth, and beside them two larger crystals that run down the sides of its head.

Summon: Use the Dread Ember at night anywhere in the world. Like other bosses it will despawn once it reaches daytime. Biome and depth do not affect the boss.

Fought: Post-Moon Lord
Sulker -> Phantasmaw -> Arterion Prime
Special?: No
Progression?: Yes

Stats:
Health:
-40,000 (65,000) (80,000) (Dread Embers)
-200,000 (385,000) (500,000) (Phantasmaw)
Attack:
-120 (200) (340) (Phantasmaw Melee)
-100 (175) (300) (Ember Melee/ Shadowflame Fireball)
-110 (188) (320) (Phantom Arcanum)
Defence:
-100 (100) (115) (Phantasmaw)
-90 (100) (180) (Ember)

[All attacks inflict Shadowflame]
[In Normal Mode Melee and Arcanum attacks inflict Vulnerability, though in Expert and Fury Mode all attacks inflict it]

Attacks:
[Normal]
Phase 1:
The Phantasmaw follows above the player, doing nothing while immune to damage. 3 Dread Embers are summoned on the ground, which each fire a Shadowflame Fireball at the player every second. Destroying these 3 embers will begin the 2nd phase.
Phase 2:
The Phantasmaw floats near to the player, launching numerous different fireball attacks. The 3 Dread Embers return in this phase, flying through the air and occasionally charging at the player, but they cannot be damaged nor use their fireball attacks. The Phantasmaw is vulnerable in this phase and has the following attacks:
-It may launch 3 shadowflames in an arc at the player up to 4 times in rapid succession
-It may launch numerous shadowflames up into the air which fall down shortly after and bounce once off of the ground.
-It may release 3 'pulses' of shadowflame fireballs in random directions. Purple energy appears all around Phantasmaw before it is converted into numerous fireballs before all being launched out from him.
Once at 155,000 (320,000) (400,000) the Phantasmaw becomes immune and the Dread Embers teleport back near to the ground; entering its 3rd phase.
Phase 3:
The Embers are the targets again, though unlike before the Phantasmaw retains its attacks from Phase 1. Each ember has a smaller flame orbiting it that does the same damage as the embers themselves do. The embers fire fireballs at the player more often than before.
Destroying the 3 embers again begins Phase 4.
Phase 4:
The Embers are airborne again; back to being immune to damage and losing their fireball attacks, but retaining their orbiting embers and begin charging at the player again. The Phantasmaw returns as the main target, gaining new attacks.
-He may summon a large number of stationary fireballs randomly around the arena which do nothing for a second before all either flying upwards or downwards. (The first is always down, then the second up, then down again, etc.)
-He may disappear; re-appearing elsewhere, and then charge at the player. During the charge he releases numerous fireballs which are affected by gravity, dropping straight down, and can bounce off of solid blocks once before disappearing.
Once at 100,000 (250,000) (300,000) health the Phantasmaw becomes immune again and the embers once again teleport near to the ground, entering their 5th phase.
Phase 5:
Phantasmaw retains his attacks, but cannot be damaged. The embers become vulnerable again and now have their fireball attacks back, as well as a 2nd orbiting flame each.
Upon destroying the 3 embers again the Phantasmaw becomes vulnerable and the 6th phase begins.
Phase 6:
Phantasmaw gains new attacks and the embers go back to their patterns from Phases 2 and 4, but now have 2 orbitals. Phantasmaw's new attacks are as follows:
-He may summon 2 'Phantom Arcanums', projectiles the size of himself, and then have them launch at the player. A couple of seconds of moving will suddenly have them stop before altering their trajectory to launch at the player again. During this 2nd charge they do not stop again until they despawn.
-He may summon a large number of fireballs to travel horizontally across the screen. Some come from the left and others from the right.
After being taken down to 50,000 (135,000) (200,000) health the Phantasmaw becomes immune again and the embers take their places for the final time.
Phase 7:
Embers now have 3 orbitals and get back their fireball attacks; becoming the main targets. Phantasmaw retains his previous attacks, but is immune to damage.
Once the embers are destroyed again he transitions to his 8th and final phase.
Phase 8:
The Phantasmaw becomes vulnerable and the embers go back to their Phase 2, 4, and 6 patterns, though with 3 orbitals. The Phantasmaw doesn't gain any new attacks, but uses his older ones with more urgency.
[Expert]
-The embers retain their fireball attacks in phases 4, 6 and 8, though slower than their previous phases' fireballs. 3 Arcanums are summoned at a time and they are faster. More projectiles are summoned in the bullet-hells and, finally, he gains one final new phase at the end.
At (45,000) (70,000) health the Phantasmaw can transition into its final 9th phase.
Phase 9:
The embers vanish and the Phantasmaw loses most of its attacks. It only retains its teleportation-to-charge attack in this phase, using it constantly. Fireballs continuously spawn from the left and right of the screen, with increasing frequency as the Phantasmaw's health lowers.
[Fury]
-More projectiles are spawned with each attack, including a 5th volley from the arc attack and a 4th Arcanum. Embers fire faster and have access to the fireball attack even in Phase 2.
-In Phase 9 the embers, instead of disappearing, begin orbiting the Phantasmaw and even teleport with him, firing at the player constantly while their own individual orbitals orbit them. More fireballs spawn in from off-screen as well.

Defeat: The embers, if active, disappear and the Phantasmaw itself swirls away into a single point before dropping all of its items.
-If the Watcher isn't already watching the player, it will appear

Drops:
-In Normal Mode it drops 2 swords, a boomerang-like disc, a spear, flail, bow, gun, rocket launcher, 2 tomes, a magic staff, sentry staff and a summoning staff. Also drops Spectral Remnant; used to craft several armour sets and a pair of wings, and finally the Relic Keystone; a material said to contain 'The Phantasmaw's power'.
-In Expert Mode it drops a Phantom Pendant which occasionally allows the player to dodge attacks, and outside of this grants buffs to damage, defence and movement speed. Also drops the Undying Fragment; used as a late-game material.
-In Fury Mode it drops the Etherial Ticket, an accessory that summons a single orbiting Dread Ember to assist the player, but as it deals damage it summons and 2nd and 3rd which also orbit the player until the player dies or teleports.

Hints:
-As the fight slowly increases in difficulty as it progresses through the phases, so memorisation of its attacks may be useful, usually from re-fights.
-As many projectiles will be present in this fight constant movement may be good to avoid too much screen clutter.
-Many of this boss' attacks deal high damage and inflict debuffs that deal damage and, on top of that, some that also reduce defence significantly. Therefore, dodging should be a priority over damage, especially in Expert and Fury Mode.

Extra Info:
-Despite having 'maw' in the name, the Phantasmaw doesn't appear at first glance to actually have a mouth.
--However, it is possible that the spikes outlining the large shadowflame in its centre may actually be its mouth, but it isn't confirmed.
-It is one of the first bosses to actually inflict Vulnerability upon the player.
[Added: 29/7/19]

The 'final boss' of Arterion and the true power of the project's titular boss. While its appearance is similar to before something feels... off.

Appearance: Arterion's 1st phase of his original fight...?

Summon: Use the True Cursed Skull at night. The item can only be obtained after defeating Phantasmaw, meaning the Watcher will be present in the background. The Watcher will disappear whilst the fight is in progress. If it reaches daytime the boss will despawn, though time appears to slow down to only half its regular speed and the background becomes purely black and thus the player cannot keep track of the moon's progress. An arena, roughly 160x140 blocks is spawned around the player, with the player at its centre.

Fought: Post-Moon Lord
Phantasmaw -> Arterion Prime
Special?: Normal Mode Finale
Progression?: Expert/Fury Mode-Only

Stats:
[Arterion?]
Health: 155,000 (240,000) (395,000)
Attack: 120 (240) (360) (All Attacks)
Defence: 100 (100) (120)

[Arterion Prime]
Health: 1,200,000 (2,000,000) (2,650,000)
Attack:
-160 (300) (450) (Melee)
-150 (287) (432) (Droplets)
-152 (290) (437) (Red Flame)
-155 (294) (442) (Red Skull, Red Seeker)
-(314) (475) (Red Death)
Defence:
-85 (90) (95) (Normal)
-500 +30% DR (1000 +60% DR) (0 +100% DR) (Hands Active)

[Only the boss' head is capable of inflicting melee damage]

[Arterion Hand]
Health: 65,000 (115,000) (175,000)
Attack:
-160 (300) (450) (Melee)
-145 (270) (422) (Red Flames)
Defence: 115 (130) (135)

[Arterion Head]
Health: 45,000 (70,000) (88,500)
Attack: 165 (308) (462)
Defence: 130 (140) (150)

[All of this boss' attacks, across all phases and including all minions, inflict Red Fury, Bleeding and Vulnerability on hit]

Attacks:
[Normal]
Phase 1 - Phase 2:
-A stronger version of the original Arterion fight. He has access to both his normal debuff and also now Vulnerability, and all of his attacks deal additional damage compared to the original fight.
Upon defeating this Arterion it will burst into gibs instead of fleeing like in the original. However, roughly 20 seconds later, Arterion's true form emerges.
Phase 3:
Arterion Prime spends most of its time slowly following the player or floating above them, based on its attacks.
-It may launch large amounts of droplet projectiles which fan out and arc as they travel and eventually fall back down to the ground. This attack is used up to 6 times in rapid succession
-Large amounts of Red Flames are summoned, all travelling horizontally either from the left or right of the player. These 'bullet-hells' usually last between 10-15 seconds, with Arterion remaining stationary throughout.
-A barrage of Red Flames are launched at the player from Arterion Prime's eyes before another 'burst' of flames are released in all directions randomly. The follow-up burst usually has between 25-30 projectiles in it in total.
-It may disappear before teleporting near to the player and charging at them
-It may disappear before re-appearing somewhere above the player, then charges horizontally in the player's general direction while raining down massive quantities of droplet projectiles down randomly.
At 80% health it transitions to its 4th phase, gaining hands akin to Skeletron's.
Phase 4:
Arterion's attack pattern changes to one very similar to Skeletron's. It swipes its arms at the player 2 times each before performing a special attack where it continuously disappears, re-appears elsewhere, and charges at the player before teleporting again up to 6 times before resetting the cycle.
-Arterion itself may perform its 1st and 2nd attacks from its 3rd Phase while not performing its special teleportation attack
-The hands, just before each teleport, release 3 red flame projectiles outwards in random directions, but lack projectile attacks when being swung at the player.
Upon defeating the hands Arterion becomes more vulnerable again, but otherwise continues the cycle.
However, at 70% health, it transitions to its 5th phase.
Phase 5:
Arterion's patterns go back to being similar to its Phase 3 ones, even regaining all of its phase 3 attacks alongside its new attacks.
-It may spawn Red Seeker projectiles from its eye sockets. These chase the player with a series of short-ranged but rapid dashes before despawning a short while later. Arterion slowly moves towards the player while spawning them.
-Arterion may continuously teleport while releasing Red Skulls projectiles. Arterion is stationary after each teleport until the attack ends. The Red Skulls are released in bursts of 8 with each teleport, all evenly-spaced, and move by travelling outwards, slowing drastically, then returning to their original speed, then slowing again, before finally despawning.
At 50% health Arterion's large body slowly falls while the head shudders in-place. After a few seconds the head breaks away before the body explodes rather violently. The head then begins to attack the player on its own.
Phase 6:
Arterion's head lacks any projectile attacks and only attacks with charges in a fashion similar to Duke Fishron's Expert-exclusive 3rd phase. It will teleport, then charge at the player between 2-4 times with long-ranged fast-moving charges, and then teleports again.
Once the head reaches 0 health the body will reform slightly below it and it will reconnect to its large body again and continue the fight.
Phase 7-9:
-All these phases are similar to 3, 4, and 5 respectively. However, all attacks are used with increased urgency and often utilise more projectiles with each attack. However, the Red Skull attack and Hand's projectile attacks don't gain any additional projectiles.
[Expert]
Phases 1-2:
-Same changes as in the original Arterion fight's Expert mode
Phases 3, 5, 7, 9:
-Additional projectiles and increased timers on the debuffs inflicted.
-Arterion may occasionally teleport during the bullet-hell attack, though still lacks free movement.
-Arterion gains one new attack where it moves to the middle of the arena and releases 4 'Red Death' attacks. These consist of massive lasers (akin the the Moon Lord's Phantasmal Deathray, but red) being released in a '+' before rotating 180 degrees around Arterion. This may be used in conjunction with a few horizontally-moving red flame projectiles or bursts of red flames released from Arterion in random directions. (Phases 5 & 9 Only)
Phases 4, 8:
-Projectiles are released from the hands both before and after the teleports, as well as 4 projectiles being released per attack. This totals out as 8 projectiles released for every teleport performed successfully.
Phase 6:
-Arterion moves faster when charging, has a shorter wind-up and wind-down time before and after each string of charges, and is is slightly transparent.
[Fury]
Phase 1-2:
-Same changes as in the original Arterion fight's Fury mode
Phases 3, 5, 7, 9:
-Additional projectiles are released with every attack and attacks are faster. Debuffs last longer.
-Red Death rotates faster and more red flames are released from Arterion whilst they are active. (Phases 5 & 9 Only)
Phases 4, 8:
-4 Hands are summoned at a time as opposed to just 2.
-Hands release red flames before and after every teleport, though the number starts at 4 it will increase to 5 when 1 hand is defeated, to 6 with 2 defeated and to 8 with 3 defeated.
Phase 6:
-Wind-up and wind-down before and after each string of charges reduced significantly. Charges speed up progressively as health lowers. Head also becomes less and less visible as its health lowers.

Defeat: The head, seeing the body begin to collapse again, shudders and tries to break out again. However, unlike before, it doesn't seem to have the strength to and thus, when the body explodes, it is caught in the explosion and destroyed. As the body explodes again Arterion lets out a single screech before its destruction.
-The Watcher will not re-appear in the sky again after Arterion Prime has been defeated at least once, even if the player re-fought Phantasmaw.
-Callie will begin to sell stronger versions of her original weapons in-place of her original ones. She will also now sell the Dreamcatcher; an accessory that revives the player with 70% health upon death with a 4-minute cooldown as well as boosting the player's damage and critical strike change by a decent amount, and Calamitous Weave; a material used to craft a few armour sets and to upgrade a small amount of weapons and accessories.
-Arterion, Sulker and Phantasmaw begin to drop Annihilation Shards in small quantities.

Drops:
-In Normal Mode Arterion Prime will drop a small amount of Annihilation Shards; materials used to craft the Annihilation Emblem and upgrade a few weapons. Also drops a sword, tome, gun and a summoning staff. He will drop the Reality Glass which, upon use, grants the player a 5-minute window where enemies may drop Purest Night; materials towards the Annihilation Emblem and also usable for upgrading a few items.
-In Expert Mode Arterion Prime drops the Night Terror Fragment; the final piece towards the so-called 'late-game crafting material' collection. He also drops the Shield of Dreams; an accessory that creates a small slow-moving orbital around the player which may destroy certain projectiles.
-In Fury Mode it drops additional Annihilation Shards and a pet 'Arterion Slime'; a Red Slime decorated to resemble Arterion Prime.

Hints:
-Remember that, much like with Supreme Calamitas, the arena will be summoned with the player present in the middle. To grant the most room the player should summon it whilst airborne and not too close to other builds.
--Or, with careful planning, the player could create their own arena to fit the proportions of the one Arterion Prime summons
-Remember that Arterion Prime takes no damage when the player is outside of his arena borders. Therefore, trying to take him out the arena to grant the player additional dodging space is actually a very bad idea.
-Phases 5, 7 and 9 are all similar to Phase 3, though with increased attacking speed and/or roster. Therefore, getting good at Phase 3 will take the others easier as well.
--The same can be said for Phases 4 and 8 which, outside of Arterion Prime's increased urgency, are also the same.
-The Rod of Discord, as expected, is great for dodging attacks. It is likely also preferable to take Chaos State damage over getting hit by Arterion Prime as the debuffs granted by Prime are devastating.
--If the player gets inflicted with Red Fury and Vulnerability they should NOT try to go on the offensive with the 5% damage increase. They should play defensively because the debuffs take away 50 defence, make the player take 10 damage per second and, on top of this, both block regen and add another 20% to any damage the player receives from attacks.
---Therefore, even a player with 200 defence could still be taking 102 damage from just Prime's contact damage in Normal Mode alone, even ignoring the additional damage-over-time effects as well.
-In Expert Mode in Phases 5 and 9 if the player cannot see Arterion or sees it heading towards the middle of the arena, stay diagonal to the boss. Once in the middle of the arena it is only a matter of time before the Red Death lasers are used.

Extra Info:
-Arterion Prime doesn't drop that many items compared to the bosses fought previously, despite being Normal Mode's final boss.
--However, this is because it instead allows Callie to upgrade her stock and allows the player to buy the weapons of their choice (with a hefty price tag, to be fair).
-The Calamitous Weave, sold by Callie upon defeating Prime, is another reference to the Calamity Mod due to its name. The Shield of Dreams dropped by Prime himself, on the other hand, is a reference to the Dream Shield from Hollow Knight.
-Despite being the Final Boss of Arterion in Normal Mode, it is only the penultimate boss in Expert and Fury mode.
-There are technically a total of 4 fake-outs during this boss battle. The first and most obvious is the fake Arterion boss at the start of the fight leading into Arterion Prime, the next when Arterion's body is defeated the first time just being a phase transition, the third being when defeating the 6th phase only to find that it in fact isn't defeated and, finally, upon its defeat which looks at first like a phase transition, but luckily is the end of the boss fight.
[Added: 3/8/19]

The true final boss of Arterion; only accessible in Expert and Fury mode. It provides a massive amount of challenge to the player

Appearance: Its body appears similar to Arterion Prime's, though fused with elements of both Sulker and Phantasmaw. Half its body shares the colours of Phantasmaw, and on that same side Phantasmaw's own face can be seen looking out to the right. The Sulker's face is seen on the opposite side, though uses its Phase 3 colours. There is a large hole where part of its ribs used to be and inside that hole is nothing but a black void. Finally, Arterion's own face in the middle of the other 2 heads has also changed; becoming greyer and less saturated in colour. His eye has changed to a more purple colour; matching the colours of the Shadowflame atop Phantasmaw.

Summon: Use the Accursed Relic. Upon use of the relic it will become 12:00 AM and remain as such until Annihilus despawns. The Accursed Relic is made using 3 parts:
-The Annihilation Emblem; made using Annihilation Shards, Calamitous Weave and Purest Night
-The Unified Emblem; made using the Fury, Guardian, Dungeon, Skeletal, Returning, Mechanical, Nightmare, Mutant, Sorrowful, Undying, and Night Terror Fragments dropped by certain Expert-Mode bosses.
-The Amalgamated Soul Fragment; made out of Souls of Fright, Light, Night, Might, Flight and Sight, Solar, Vortex, Stardust and Nebula fragments, Luminite Bars, and Purest Night
The boss will only despawn once it or the player is defeated.
Upon summoning the boss an arena roughly 200x160 blocks is made with the player in the centre, in a similar fashion to Arterion Prime's arena.

Fought: Post-Moon Lord
Arterion Prime -> Annihilus
Special?: Superboss, Expert-exclusive, True final boss
Progression?: No, finale.

Stats:
Health:
-(5,000,000) (10,000,000) (Annihilus)
-(50,000) (75,000) (Mini-Sulkers, Orbiting Sulkers, Dread Embers, Arterions)
-(200,000) (400,000) (Sulker, Phantasmaw, Arterion Prime, Bone Meals)
Attack: (Instant-Kill)
Defence: (50)

[All of this boss' attacks will instantly kill the player, regardless of current health.]
[Annihilus' upper body and head are capable of dealing contact damage, but his lower body is unable.]

Attacks:
[Expert]
Phase 1:
Annihilus has most of the attacks used by Sulker, Phantasmaw and Arterion Prime and can use most of them at any point during the battle.
-It may use Sulker's Ghostly Energy, Tear Bolt barrages, and Spectre Fireballs, as well as occasionally summoning Red Mini-Sulkers which charge at the player, teleport, and then charge again.
-It may use Phantasmaw's Shadowflame arcs, barrages, pulses, stationary fireballs, bullet-hells, and phantom arcanums.
-It may use Arterion Prime's droplet arcs, red flame bullet-hells, red flame barrage and bursts, droplet rain, red seekers, red skulls and red death attacks.
At 85% health Annihilus will become transparent and invulnerable to damage, but also becomes both unable to damage the player. He transitions to his 2nd phase.
Phase 2:
3 Bone Meal worms are summoned. Annihilus will remain behind the humanoid one, but does nothing more than following the position of the head. All 3 worms have the same health and defence, and all instantly kill the player on contact.
-All 3 worms use their usual behaviour when attacking, despite the stat changes.
Upon defeating the 3 worms Annihilus will become the target again.
Phase 3:
Annihlius goes back to being able to move and deal/take damage, as well as losing its transparency. It gains a single new attack where it disappears before re-appearing new to the player and charging at them; an attack that Phantasmaw and Arterion Prime had access to.
At 70% health it becomes transparent and invulnerable again, this time summoning a new boss.
Phase 4:
The Sulker is spawned in, with Annihilus following the Alpha Sulker's movements. The Sulker acts the same as in its own fight, but has altered base health for both the main Sulker and for its minions summoned throughout.
Upon defeating The Sulker Annihilus will change to its next phase.
Phase 5:
Annihilus goes back to normal again and goes back to attacking the player. It doesn't gain any new attacks outside of now spawning stationary Red Flames during the charge, however, instead just relying on the attacks from its previous phases.
At 55% health it transitions to its 6th phase.
Phase 6:
Annihilus, unlike other instances, doesn't actually become transparent and stop attacking in this phase, but instead continues his assault whilst invulnerable. 8 Dread Embers are summoned, randomly scattered across the arena, which need to be attacked to continue the fight. Each one fires fireballs at the player with increasing speed as their health lowers.
Once all 8 Dread Embers are destroyed Annihilus transitions into Phase 7.
Phase 7:
Annihilus becomes vulnerable again and gains additions to some of his attacks.
-Attacks now have a chance to consist of additional projectiles, and attacks are also launched with a slightly increased frequency.
At 40% health Annihilus becomes transparent again for the next phase.
Phase 8:
Phantasmaw is summoned, though only uses behaviour from its first 2 phases. It summons Dread Embers which fire at the player, then begins to attack itself once they are destroyed. The Dread Embers aren't summoned again, though Phantasmaw has access to all of his attacks as soon as he becomes active. Annihilus follows the Phantasmaw's movements once it becomes active.
Upon defeating Phantasmaw, Annihilus becomes vulnerable and opaque again.
Phase 9:
Annihilus' attacks don't change outside of its charge attack.
-At the end of the charge, Annihilus releases a burst of slow-moving Shadowflame fireballs outwards in all directions. The amount can be anywhere from 8 to 20.
Upon taking Annihilus' health down to 25% it transitions to Phase 10, becoming transparent and invulnerable again.
Phase 10:
3 different Arterions are summoned at the same time. Each one of them uses their normal attacks, but their attacking speed and movement speed are reduced, with the charging speed reduced even further. Annihilus follows a random Arterion, though switches to a different one if its original one is destroyed.
-While they are active Red Flames occasionally move horizontally across the arena from either the left or right. The frequency of their spawning increases with each Arterion defeated.
Upon defeating all 3 Arterions, Annihilus becomes opaque and vulnerable again. The red flames stop spawning.
Phase 11:
Annihilus doesn't gain any new attacks and, if anything, appears to slow down its attacking frequency back to what it was at the beginning of the fight up until Phase 5.
Annihilus becomes invulnerable and transparent again at 10% health, transitioning to its 3rd last phase.
Phase 12:
Summons Arterion Prime, who starts out in Phase 5. Its attacking speed is slightly reduced, however. At 10% health it transitions to Prime's 6th phase; with the head charging at the player. However, the head is slightly slower movement-wise than in its original fight. As with the other bosses, Annihilus follows Arterion's movements.
Once Arterion Prime is defeated, Annihilus becomes opaque and vulnerable again.
Phase 13:
Uses its Phase 11 attacks, though appears to have slowed down even further now. Very occasionally either a red flame or shadowflame projectile will spawn and move horizontally across the arena. The frequency of these fireballs gradually increases as Annihilus' health dwindles.
Finally, at 5% health, Annihilus transitions to its final phase.
Phase 14:
Charges at the player, teleports, and charges at the player again; repeating the cycle. It can only spawn projectiles either from the charges (established in earlier phases) or the horizontally-travelling fireballs which still increase in frequency as Annihilus' health wanes.
-Annihilus' charging speed actually slows down at lower health

Defeat: Annihilus will stop, and lose the ability to damage the player completely. Slowly, its body will disintegrate, starting from the bottom and slowly making its way to its heads. Arterion's head is the only thing left over after the body disappears, though the purple glowing eye has faded. Arterion's skull turns completely grey and then falls and, upon hitting a solid block, bursts into small gibs. As with Arterion Prime, the arena will disappear, but also time will go back to flowing properly.

Drops:
-In Expert Mode it drops Reality Weave; used to make a large assortment of items (though most of them are vanity items and not useful for combat). Also drops the Crimson, Phantom, Weeping and Destruction Monoliths which represent Arterion, Phantasmaw, Sulker and Annihilus, respectively, though are simply furniture decorations.
-In Fury Mode it also drops the Fury Gel, which summons a small version of The Fury as a pet, and also the Chaotic Fragment which, while it does nothing on its own, has the description: "Full of unstable energies, always flowing. Not useful alone, but perhaps part of something far larger..."

Hints:
-A lot of the things mentioned in Arterion Prime's fight also apply here as well. The player could make an arena inside of the forced arena to give them an advantage, has to be aware of the arena's summoning positions.
-The main thing to remember about this boss is that it is simply an endurance test, and thus doesn't need to be defeated. All of its drops are simply vanity-based and thus aren't too important. Keep calm, take breaks, don't let Annihilus' difficulty consume you.
-Accessories that provide revival effects are very useful in this fight as they can give the player another chance as the boss otherwise kills the player outright from any attack. Combining these accessories can also be useful for the ability to take multiple hits.
-Practice on The Sulker, Arterion, Phantasmaw and Arterion Prime before fighting this boss as the player will have to rematch them during the fight anyways, plus Annihilus' movepool is mostly consistent of their attacks or ones similar to them in function.
-The fight is extremely long, so using autopause to take breaks may be beneficial. This can also grant additional time to plan how to deal with new phases or incoming attacks.
Extra Info:
-This fight isn't intended to be won by everyone, due to its excessive damage output. Therefore, the few items locked behind this boss are all vanity-based and only useful for status symbols (or completing collections).
-The Chaotic Fragment dropped in Fury Mode is a reference to the Fragments used to craft Annihilus' summoner, as well as the Chaos Doppelganger from Boss Ideas 4 & 5; the main antagonistic force in both pages.
--This, combined with the strange black energy leaking from the crack in Annihilus' head and seen within its eye sockets and body, may mean that Annihilus has something to do with Chaos in some respect.
---The Chaotic Fragment's description both references the few animated sprites seen on this page ('always flowing'), as well as the hints towards further projects in the future ('perhaps part of something far larger...')
-Annihilus, unlike other bosses, is actually just an amalgamation of bosses as opposed to its own individual entity. It has the body of Arterion Prime, though also merged with both The Sulker and Phantasmaw; two other Post-Moon Lord bosses. Along with this, it summons Bone Meal worms after taking damage as well and has a pet version of The Fury, connecting it to all bosses except the Desert Defenders directly.
[Added: 3/8/19]

Unexpectly quick, considering I made it sound like I was going to disappear under-the-radar for a while on the last part of Boss Ideas 5, eh?

Well, that was Arterion; a collection of bosses very different to the ones I usually design. Lot more edge, lot more Calamity References, lot more... uhh... okay I wasn't sure where I was going with this to be fair, whoops

Anyways, as usual I hope you enjoyed the designs here, even if they were 'edgier' than my usuals :P


[This page wins the Outmemed Medal for almost reaching #1 on trending, though losing to a meme page]