This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Kingdom of Loathing
Invoked: Nightmare Fuel is the basis of Spooky damage.
Everything about the Guy Made of Bees. To be specific: The Guy Made of Bees is the kingdom's answer to Candyman. He was an accountant who quit after endless years in a dead-end position and took up beekeeping. The people who knew him noticed that as his visits became less and less frequent, he spoke more and more about the wonders of bees... while his speech became more and more buzz-like. One day he stopped coming at all. And then people started to vanish. When they burned down his apiary, the bees rose from the flames in a human-like form, and now all they do is hate...
"WE ARE BEES. WE HATE YOU."
The four-shadowed mimes. Not only were they wandering monsters and thus able to be fought in any zone, they stood in stark contrast to the usually-even-more-hilarious-than-usual content of April Fool's Day. The suddenness was jarring.
Most of the Jar of Psychoses zones unlocked by the Angry Jung Man are merely weird, with plenty of the game's trademark humor. Then there's the Crackpot Mystic's Psychoses, a Mega Man-inspired zone where you fight the embodiments of the Mystic's Anger, Fear, Doubt, and Regret, which are full of monsters that taunt you with their respective Adult Fears. Any jokes here are tinged with Black Comedy and Surreal Horror.
The first few parts of the Sea Monkees' questline are the usual silly fun, but when you set out to rescue Mom Sea Monkee, things take a sudden turn into horror. The Caliginous Abyss has you fighting Eldritch Abominations that look somewhat reminiscent of the later levels of Eversion, and there are ominous sights and messages at the end of battles. Monsters in this zone drop items that work as buff potions, usually with a degree of temporary Body Horror. The descriptions of both the items and their effects are short and unsettling, in contrast to the usual humorous messages.
Things don't get any better after you complete the quest. Mom Sea Monkee is... off. Her behavior is violently unhinged, with nary a trace of the usual KoL humor,Little Brother seems to be hiding something, and Grandpa doesn't seem to realize Mom's back if you ask about her (although that last part may just be an oversight by the devs.)
There's also fighting Dad Sea-Monkee and the events surrounding it. Much like rescuing Mom Sea Monkee, they're devoid of any humor, instead having unsettling, cryptic messages. Heck, one of the hit messages is "You blink, and when your eyes open you are on the wrong side of the window. The air is sucked from your lungs. Blood sprays from your eyes and ears. You scream, but, well, you know."
Even Monster Manuel, who still managed to have lighthearted quips regarding The Caliginous Abyss monsters and the two Elder Gods, has nothing humorous to say about Dad.
The Twin Peaks level is probably one of the most frightening locations in the game, being mostly devoid of humor outside of the hedge maze encounters and filled with a pretty grim atmosphere. Not meeting the fairly steep requirements to solve the various puzzles can result in horrifying scenarios were the player is molested and chased by an angry undead corpse woman, trapped in a large food pantry, or stuck following ghostly music into a creepy painting. Only to end up stranded back outside in the cold after each adventure ends and forced to make their way back through the hedge maze yet again. Granted, knowing a little about theworks being parodied helps take a bit of the edge off, but not much.
Many of the Dreadsylvanian monsters have grim descriptions apt for the tough zone, especially the Spooky monsters.
Further up the castle hallway, you spot a skeleton shuffling around in a tattered and threadbare floral-print housecoat. You ready your weapon and move forward to attack, and as the skeleton senses your presence, it whirls around. The housecoat flaps open. The skeleton has another, smaller skeleton hanging just below its ribcage. Like the skeleton of a monkey, or... or... a doll. Yes, that must be what it is. A doll skeleton. Which is jeering and clacking its little teeth at you.
Most of the Zombie Slayer path is funny, but some of the descriptions are unnerving. Most notable is the Ravenous Pounce skill (an attack that's more powerful the lower your Fullness), which is described as follows:
The Spookyraven Manor revamp in May 2014 added tons of new content, including two new sidequests, the Lady Spookyraven quest and the Lights Out quest, which was itself into two parts, Elizabeth and Stephen. The Lady Spookyraven quest involves the usual monster-fighting and fetch-questing shenanigans, and Elizabeth's half of the Lights Out quest is fairly tame, but Stephen's quest is downright disturbing. First, you discover that part of the reason his pets kept dying is because he practiced surgery on them as a kid. You later find Stephen took up taxidermy, and some of his later experiments got... weird. Things get even weirder when you find a hidden laboratory with signs that Stephen tried this on living animals, and eventually succeeded by transplanting a brain from somewhere into a wolf's body. Eventually, you find a code to a machine in the laboratory on the third floor, and find out what happened to Stephen: he fused himself with his "patients" so they could be together again, and you just brought the resulting belligerent mish-mash of animal and human parts back to life. Whoops.
TOGETHER! TOGETHER FOREVER!
The adventure that nets you the newly-hatched mayonnaise wasp: Your character continually flips between noting increasingly worsening symptoms and an unhealthy obsession with mayonnaise, followed by a gruesomely-described Kingdom of Loathing take on the Chestburster scene from Alien.
A 2017 event had an Eldritch Abomination named Sssshhsssblllrrggghsssssggggrrgglsssshhssslblgl invade the kingdom.