Dungeon Crawl/YMMV

""Did you find something to kill? -> Is it Sigmund? -> You poor bastard.""
 * Breather Level:
 * The Lair. While it contains some powerful threats of its own, a character that hasn't been too fond of Dungeon Bypass typically has no problem clearing the branch as soon as they find it. The Lair's sub-brances are far from being easy, though.
 * The dungeon levels between The Lair and The Vaults typically don't progress in difficulty as fast as the character gains power, making them fairly easy to clear. Further emphasized as many players clear The Lair and Orcish Mines before going that deep.
 * Demonic Spiders:
 * Centaurs. They're fast and they have a powerful ranged attack.
 * Unseen horrors. They move like bats, hit hard, and they're invisible. Their invisibility is particularly nasty for lightly armored characters, who cannot take advantage of their mobility to dodge the monster's attack.
 * Any sort of higher demon, but especially any demons marked with '1'. Second-rank demons (marked with '2') will just give you really bad mutations, summon hordes of minions and set you on fire. First rank demons are the Fiends and the Executioner. The Executioner will make itself ridiculously fast and proceed to hit you like a freight train. Fiends have a (nearly) unresistable spell that instantly cuts your current HP in half. While throwing fire bolts and poison arrows, and summoning hordes of minions, who then summon hordes of their own minions.
 * Game Breaker: The Storm spells, especially Fire Storm. Both spells deal damage comparable with Crystal Spear to multiple enemies, both are partially irresistible, and both leave damaging clouds behind. Fire Storm takes it further by having Smite-targeting (if you can see something, you can hit it, regardless of what's in the way) and summons fire vortices to distract survivors. If you worship Vehumet and wield a staff of energy/cast Necromutation, then you can basically cast it for free.
 * Nemelex Xobeh also qualifies if you know what you are doing. The legendary versions of his decks, along with high piety, a decent envocation skill, and liberal use of his gifted abilities essentially turn any PC (even purely melee focused characters) into an epic level mage. Capable of summoning literally dozens of creatures, in combat, with a zero chance of being hostile for a paltry 2 mana or blasting several of the strongest spells in the game better than an equal level wizard could. Not to mention powers possessed no where else in the game, such as the ability to create food out of nothing. On top of all this old Nemelex has a grand total of one real rule (using combat oriented cards when there's no one hostile around), meaning the only real way to get excommunicated is to stand around for tens of thousands of turns with out sacrificing anything to him or using cards in combat. Something a player is never going to do.
 * Genius Bonus: There is a reference to The Metamorphosis in the description for the giant cockroach monster.
 * Goddamned Bats: Totally true of Crawl's bats. They barely even hurt you, but hit multiple times in a turn, and have so much evasion they're hard to kill (most annoying is that they're so fast they can move toward you, hit you, and move out of reach in one turn, meaning that you can't melee them unless you make some odd moves). Potentially deadly in the early game, mildly irritating afterwards.
 * Ugly things and slime creatures later on in the Dungeon. They always appear in packs, are faster than most PCs, and absorb a good chunk of damage before dying or splitting. Also see the Demonic Spiders entry above for a description of the Invisible Horror.
 * And all of the above creatures run away when damaged. Slime creatures have incredibly annoying habit of fleeing and coming back within seconds, having completely regenerated in the process.
 * Crimson Imps (or just "Imps" in earlier versions) have a trio of nasty abilities: they're hard to hit, they blink around a lot and they regenerate lightning-fast. Luckily, they're not usually dangerous on their own - but if they find a strong weapon, they can become Demonic Spiders for low-level characters, particularly mages.
 * Memetic Badass: From the "Foolproof Plan for Winning Crawl" flowchart:


 * Nightmare Fuel: If you have a good imagination, there could be plenty. Using Animate skeleton on a fresh corpse is one such case ("Before your eyes, flesh is ripped from the corpse!"). Another one is when you make an enemy bleed with your sharp claws and they proceed to run around the dungeon painting it red.
 * That One Boss: Crawl has enough for all stages of gameplay. Lucky characters may even live to encounter them all!
 * Sigmund is a human wizard Magic Knight  nutcase Badass notorious for killing off a lot of early level characters; he wields a scythe, which at this point in the game is a powerful weapon (although it’ll suck when you try to use it), and can become invisible, cast confuse on the player and throw fire. Most players will avoid or escape him if they meet him, unless they have an edge, like a wand or potion.
 * Grinder is another early game That One Boss. He's a shadow imp who can paralyse the player and cast a nasty pain attack. Grinder typically appears just a bit later than Sigmund, and the two are joked to have a rivalry going on to see which can claim more lives of new characters.
 * Nessos the centaur. Even normal centaurs are fairly bad because of their speed and ranged attack, but Nessos' attack is even more powerful and he has the ability to blink away from the player to make killing him in melee harder.
 * Mennas the Religious Bruiser is a late-game That One Boss, particularly against spellcasters. The worst part about him is his ability to silence his surroundings, making spellcasting impossible. He's also very fast and very strong in melee.
 * Maurice. A thief that'll haste himself, turn invisible, then sneak up to you and steal your stuff. After blinking away he'll turn your wand of draining against you whilst quaff your half-dozen potions of curing.