This guide really isn't much of a guide. It's more of a random collection of facts I haven't seen mentioned anywhere else. To drive that point home, here's a bunch of info I couldn't fit into their own sections:

• Tools can pluck torches off of walls
  The difficulty will almost always be "easy" or "very easy" even at 0 nimbleness, so carrying some tools regardless of class is always useful. This not only gives you a usable torch, it also removes a light source from the environment, which could give you a hiding spot if you need it.
  
• Not all sprites are created equal
  Watch out for rare white sprites that are brighter than the others. Normal sprites will only let you see in a small circle around you. These better sprites meanwhile will light up most of the screen and will repel shadow imps. I could've sworn I've bagged a really bright yellow one, but that was before I took note of map seeds, so don't take my word on that.
  
• Myst + ring/amulet of light = bad
  If you ever get an accessory that gives off magical light, unequip it whenever you enter any myst. Having the light on near myst sometimes causes it to fire off a magic missile in a random direction that can set off more myst for a deadly chain reaction. I was wondering for the longest time where those missiles where coming from...
  
• Magic weapons and weapon coating
  Coating your weapon with oil/holy water/myst water also (unintentionally?) reactivates any drained magic effects for hits that use up the coating. Eg. drained vampiric mace + holy water will cause the mace to restore health again for 3 swings if you hit a skeleton.
  
• Sigils from non-bosses
  Some of the more powerful, non-boss enemies actually have a small chance to drop sigils based off of them. These are pretty rare, but here are the one's I've personally seen:
  
  • Water Elemental - Water
    
  • Fire Elemental - Fire
    
  • Earth Elemental - Earth
    
  • Flesh Golem - Life (Praying to Grievous while fighting enemies can cause these guys to spawn infinitely. This makes life sigils somewhat reliably attainable if you're doing a Grievous run.)
• Grab all the plants
  Most plant effects will be activated by simply walking over them.. effects like giving off light, exploding, or slapping you with magic. Some of these plants are much more useful when you carry them with you or throw them at enemies though. To grab plants, just approach them with a free hand and click. Keep an eye out for these. They're basically renewable bombs/light sources for the current floor.

Ever pick-up a staff of summoning, or a staff of gas and you just threw them out to save on inventory space? Not anymore! You can actually combine these staves into better staves to consume them and pass their enchant level to the more useful staff. Simply place both staves into a forge and combine away. There's even an in-game book that mentions this.. but it's surprising how rare this actually gets mentioned around here.

There are some rules to follow when combining though...

• The order of the staves DO matter when combining
  The staff on the top/left square when forging will win out if the staves are equal level. Summoning^1 + MagicMissile^1 = Summoning^2 ... But MagicMissile^1 + Summoning^1 = MagicMissile^2
  
• The higher level staff will always win out when forged with a lower level staff
  ... but there does seem to be a bug with the math depending on the order: Summoning^2 + MagicMissile^1 = Summoning^3 as expected ... but I swear I once got Summoning^4 when I flipped the order. Cause is still unknown, but note that this doesn't always happen.
  
• A staff cannot go past its highest enchant level
  Most staves only go up to ^5. Any levels past the maximum will be left over on the other staff. Instead of getting destroyed, the other staff just gets downgraded: MagicMissile^4 + Summoning^2 = MagicMissile^5 + Summoning^1
  
• The resulting staff (or staves) will always have full energy
  Just as with an enchant or recharge scroll, you might want to empty out both staves first before combining so you get maximum usage.
  
• Staves of Gas/Poison/Flames/Cold count as ^1 for the purposes of forging.
  They also can't be enchanted to a higher level, so using one of these staves for forging will ALWAYS destroy them if applicable.

Now get out there and maybe stop throwing away those extra staves...

Altar and prayer mechanics are kind of confusing and probably one of the buggier parts of the the game tbh. I've tested these to the best of my ability, but feel free to correct me if I put in something wrong.

Altars accept stuff, give something in return, and raise favor for the god involved. Sounds simple enough.

AFAIK, bjorndadwarf's section here from his/her guide is still accurate enough. Just get the possible offer-reward conversion from here:
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=1337832168

• Offer in Descending Order
  For some reason, altars will only reliably give you something in return if you offer in DESCENDING ORDER. For example, with Sophie, if you offer life potion (t3) -> tea (t2) -> ration (t1) in that order, the altar will reward you three times as expected. But if you go tea (t2) -> life potion (t3) -> ration (t1), the altar will only give the rewards for the tea and the ration. This is probably a bug. That's a wasted life potion.
  
• Strictly once per tier
  Each separate altar will accept and give a reward ONCE PER TIER. Sometimes an altar will allow you to offer a certain tier infinitely and keep taking your stuff, but the rewards will only ever come for the first offering for that tier.
  
• Grievous's Kill Count
  The kill count displayed when right-clicking a weapon isn't actually accurate for the purpose of offering weapons to Grievous. He seems to ignore kills for weaker enemies. Fodder enemies such as rats, bats, ants, wasps, fish and basic spiders don't appear to count.
  
• Sophie vs Grievous
  Sophie's altars might not give rewards if you've previously offered to Grievous. The opposite doesn't seem to be true though. Grievous still always seems to give out stuff regardless of favor with Sophie.

Prayers offer situation-dependent effects that can help or hinder you in your journey. Of the 3 gods, Sophie seems to be the most easily annoyed and least reliable. She doesn't take anything when you pray, but you can really feel the favor loss as it's very unlikely that she answers another prayer for a while. Density meanwhile just takes gold. Lastly, grievous summons monsters that seem to get tougher the more floors you've discovered.

• Near door / stuck somewhere:
  All of the gods will open any NON-PUZZLE door for you if you pray near it. Some spinning stars will appear near the door, and stepping on it for a bit will unlock the door. Additionally, this prayer will also work when the game thinks you're trapped, eg. on an island surrounded by lava. In this case, the spinning stars will teleport you somewhere else on the current floor.
  
  Tip: Being in favor with grievous allows you to power through door after door as long as you can fight his summons as there seems to be no favor loss when praying to him.
  
• Near monsters:
  Grievous:
  Enrages(+speed and +strength) you for a pitiful amount of time ~15seconds, and summons more monsters. You probably don't want to do this unless you want to raise your weapon kill count as an offering back to him, or if the monsters summoned drop something you want.. like flesh golems which have a small chance to drop life sigils.
  Density:
  Turns you invisible. Same duration as invis potion.. which is pretty long. (He doesn't seem to take any gold or lose favor when doing this though. bug?)
  Sophie:
  Instantly teleports you to somewhere else on the map + favor loss.
• When on low health:
  Sophie: Full Heal + favor loss

If you've ever gotten the Amulet of Yendor before, you'd know that going up a few levels with it in your inventory will cause the Guardian of Yendor to spawn and try to kill you all the way until you escape on the top floor.

This guy's probably one of the toughest enemies in the game with his huge health pool, ^3 random enchant greatsword+shortsword, and ability to cast lighting. Even worse, he's immune to non-enchanted gear and will share any damage+effects you do to him when in melee range.

Still, he's not invincible, and killing him will make the trek out of the dungeon much, much easier, along with giving you a couple of cool swords to kill any remaining enemies with.

• Do not fight him when he first spawns
  The first time the guardian spawns, there's a small chance that he moves 10x faster than normal. Dealing with him in this (bugged?) state is going to be difficult as his lightning spell is going to be near-instant cast. This goes away when you move to a different floor though. So once he spawns for the first time, if it looks like he's moving crazy fast, just go back down the way you came from (or use a descend scroll) and just go back up again before you fight him.
  
• He can transfer damage+effects if you stand too close
  The damage+effect transfer is range based, not weapon based. This even includes any heals you get for a weapon of vampirism, for example. He gets healed too! A greatsword/spear isn't long enough to get past this transfer range, but whips/throwing weapons/ranged weapons are fair game as long as you stand far enough away. I repeat.. STAY BACK.
  
• Enhanced weapons only
  Contrary to what the wiki says, he's not immune to arrows. The bow doesn't even have to be magical to do damage. As long as the weapon is at least +1, it WILL damage the guardian.
  
• Chain stunning with Staff of Stunning/MarineMissile
  The guardian can be chain stunned, and it's the best cheese strat you have to kill him. Although the Staff of Stunning(earth sigil) can potentially stun for longer per use with higher enchant levels, the Staff of MarineMissile(water sigil) will always have much more uses while also doing damage at the same time.
  Simply stun the guardian into a corner and spam your whip/throwing weapon/bow from a decent distance while he's stunned. Just don't forget to cycle back to your staff to stun him again before it wears off though.
  
• The unseen cheese is the deadliest (and slowest)
  If you have nothing to stun him with, stealth is also a safe (but tedious) option. Here's how that works:
  
  • The guardian can't see through stealth.
    
  • If the guardian hasn't seen you for a while, he'll teleport to somewhere else on-screen and move around randomly.
    
  • Using throwing weapons and whips (but not bows) will not reveal you if you're hidden. Spam them if he happens to teleport close enough to you.
    
  • Eventually he's going to teleport away since he still doesn't really see you.
    
  • Pick up any throwing weapons on the ground if he teleports somewhere far enough away.
    
  • Hide before he gets back.
    
  • Repeat for a ridiculous number of times.
  
  Though... if one of the guardian's random weapons has a glow effect that lights you up when he gets close(fire/purity/etc), you're boned. This can still be countered by throwing a potion of drain magic to get rid of the enchantment, or a cloak of camouflage(cloak+earth), which lets you hide regardless of light level.

Happy hunting. And yeah, yeah.. I know the conjurer is probably tougher, but I haven't encountered him enough times to formulate a cheese strat yet.

I'll probably add new stuff in here when I discover them. Feel free to comment if there's something wrong. I'd be happy to retest and give credit when appropriate.

I'm not really sure how to end this. As I said, this guide is as unstructured as it gets. Now, enjoy the abrupt conclusion.