NetHack/Nightmare Fuel

You see here a murky potion.
s - a murky potion.
What do you want to quaff? [stvwx or ?* ] i
Ulch! That potion tasted foul...
You fall into a deep sleep!
Your dreams are polluted by disturbing revelations...

You're probably thinking, "What could possibly be scary about a game made of numbers and letters?" Good question! While not geared as heavily towards the spooky as most other roguelikes, NetHack still has surprising frights to offer. Why not take a gander for yourself?

Setting

The idea that you can eat any sapient or sentient being you kill as long as they're not the same species as you is... harrowing to contemplate at length, especially considering that no one actively takes issue with you eating a fresh kill in front of them.
There are lots of "used armor" shops scattered throughout the dungeons. Sometimes these shops contain cursed armor, which can't be removed except by certain methods such as uncursing it. Now where do you think the shopkeepers got their inventory from? Dying while in a shop confirms it outright - the shopkeeper takes all your possessions in that event (which amounts to making them part of the shop's inventory). Seems reasonable enough... but if you directly steal from the shop (e.g. teleporting out with unpaid items, digging out...), the shop owner will try their damnedest to chase you down. If you should die after that point, guess who comes for all of your loot?

Monsters

Master liches and arch-liches are Demonic Spider (if not Boss in Mook Clothing) levels of frightening to encounter in normal gameplay, and it's even worse when a monster's polymorphed into one (which occurs with shapeshifters and polymorph traps that can spawn below dungeon level 7). Imagine you finally make it to DL 10 after a long and arduous series of splats, and as you're exploring something invisible warps towards you and starts destroying your armor, cursing your inventory - or worse, priming the infamous Touch of Death. Even if it isn't nearly as likely as other outcomes, the fact that something can potentially teleport to you and render you a corpse with no warning will leave you double-checking every move you make.

Oh no, she's using the touch of death!
Do you want your possessions identified? (y/n)

Chameleons - in real life they're bizarre and unusual animals, but generally no threat to the average person. In NetHack? They're Hollywood Chameleons that can randomly turn into one of several eligible forms every few turns or so - including several powerful forms that would be completely out-of-depth for a player otherwise! If you're especially unlucky, you might just encounter one in the form of the aforementioned master/arch-lich above...
The trolls found within the dungeon are incredibly strong and persistent for midgame foes - not only do they frequently generate with polearms to swat you with if you try to put another monster between you and them, but they have a nasty melee-range bite! To make matters worse, then have Regenerating Health abilities that put werecreatures to shame. Even if you manage to put them down, their corpse will often revive as soon as possible, restoring it to full health and likely leaving any adventurer that struggled to defeat it just once at its mercy. If you're not prepared for them, your best option is to pull out the wands and spells, or else just run.
- Even disposing of the corpse isn't always guaranteed to get rid of it: it's incredibly heavy, meaning you'll have to lug a lockable container over if you want to stuff them in, and spending turns carrying one heavy object to the other just increases the risk of it coming back to life. Eating the corpse seems most intuitive, but also carries the risk of it resurrecting with each turn spent, right down to the very last bite. (Strangely, carnivorous pets don't seem to have the same issue.) It's entirely possible to satiate yourself from low nutrition trying to finish a single troll corpse! Out of the other "permanent kill" methods - stoning, disintegration, drowning, tinning, etc. - most of them might not be an option unless you have specific items, so you'd better hope the Random Number God is smiling on you.
The demon lords and princes in Gehennom more than earn their name, but Asmodeus and Demogorgon are likely among the scariest of the bunch.
- Asmodeus is among the few demon rulers that you can bribe in order to avoid fighting - sure, you can also reject his demand, but if you lack resistance to cold damage... well, did you know he has a unique cone-of-cold attack? Did you know he can use it at close range? Did you know this close-range version can immediately frost you with high enough rolls if you lack the proper resistance? It's by far the nastiest spell in the game, and if you don't have a way to tank it... maybe you'd better take that deal.
- Demogorgon meanwhile has no such inclination - the second he shows his faces, he'll immediately begin tearing into you. Demogorgon only ever shows up if another demon or caster summons him; if things have gotten that far, you either went looking for trouble or you're in way over your head. Demo himself has disease-inducing tentacle attacks, which also stun you if the first hit already made you sick, and he can of curse your items like most hostile spellcasters - pray he doesn't curse that unicorn horn you were relying on!
The Wizard of Yendor - a persistent and nigh-immortal warlock that will chase you to the ends of the earth if you take the MacGuffin he believes to be rightfully his (and might have been using as a Soul Jar to boot). Zap him with a wand of death? Congrats, ya got him - hopefully you didn't waste too many shots because he always comes back, in a new body, one level stronger and ready to menace you again. Trying to fell him in pitched combat? Oops, he stole an artifact, now there's two of him, and they're both disguised as nasty monsters! Were you relying on that quest artifact for magic resistance? Sure sucks to be you! Thought you could escape him by leaving the dungeon and heading for the Planes? Think again - when we say "ends of the earth", we mean it!

Other characters

You. Yes, you, the corpse-eating, gold-stealing, magic-abusing One-@ Army! With the blessings of your god, you'll be wreaking havoc upon the dungeon and its denizens. Sure, most of them are trying to kill you first, but what of the ones who aren't - did that peaceful dwarf deserve to be robbed and killed just because you really wanted to have their pick-axe? Is it worth kicking every d you come across? Even without leaning heavily on moral angles, the sheer amount of killing and eating (usually involving the things you just killed) that you have to do would be thoroughly off-putting in a lot of other contexts.
- And that's before the discovery that you're War of the Four Horsemen. Attempting to tin the corpses of the other Riders gives a response that "War does not preserve its enemies". If the Riders were your 'enemies', then what were all the other monsters you faced? Obstacles? Inconveniences? Or were they just stepping stones as part of a test for you to come into your true self?

You jolt awake with a fright!
In your panic, you trip over a cockatrice corpse.
You turn to stone.
Do you want your possessions identified? [ynq] (n) _

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