Please don't list this on a work's page as a trope. Examples can go on the work's YMMV tab.
Nightmare Fuel: Fallen London
Fallen London is a Twitter-powered, largely text based game with clever storylines and and interesting steampunk-ish atmosphere. Some of the descriptions, however, are pure Nightmare Fuel.
What are the Sorrow Spiders? The story goes that spiders drink from your eyes while you sleep. Sorrow-spiders bite off a whole eye. They get their name from the tears that flow from the remaining eye.
Oh, and they don't steal eyes to eat them. They use them as eggs.
If your Nightmares quality reaches five, you will get this event: In the street, you pass a tall, cheerful man with a brisk manner, a stovepipe hat and a row of bright brass buttons down the front of his coat. He winks familiarly as you pass and spreads his hands: eight fingers. You've seen him before. Of course you have. He was beside your bed when you woke this morning.
And this storylet follows you everywhere you go. Except into death, exile, or prison.
And the 'failure' of the particular event: 'That night you dream of a tall building lit by cheery fires. A sign reads CLEFT FOR THEE HIDE IN ME. The walls are wrong. The walls are wrong.' Quite possibly one of the most disturbing lines in the game.
Also the Cantigaster. Everything about the Cantigaster. The Cantigaster basically is a mass of poisonous flesh - that Was Once a Man, he got that way due to a Deal with the Devil made by his then-fiancee, the Duchess, after he was bitten by a snake. His life was saved but at a horrible price.
Everything about the Iron Republic. Everything. You can see it right from the description:
Freedom from laws and tyrants. All laws and tyrants, without exception. This place is exceptionally dangerous. You may be permanently changed.
Some of the recurring dreams storylets can be rather disturbing, especially the Is Someone There? collection.
You are standing between two mirrors. Your reflection smiles, so you smile. Your reflection moves its hand, so you move yours. It takes a very long while for you to realise that this is the wrong way around.
The "dream about fireworks" you can get if your lodgings are rat-infested:
You dream that you're standing, holding a candle, in a classroom filled with cheerfully shouting children. You touch the candle to their heads, one by one. Each time you do, their skin crisps away to reveal a child-sized mass of rats. Awake. Awake.
The Mr Sacks visits. Ever wanted to see some of the Santa mythos through a spectacularly creepy, original lens?
That night as you sleep, Mr Sacks crouches on your window-sill. His robe is the colour of salt, and his hood is trimmed with red fox-fur. I have brought you gifts, he whispers.
Especially when you realize that particular Mr Sacks is Mr Eaten.
When your Nightmares gets to 6, you start to draw Recurring Dreams cards that cannot be discarded. The only way to get rid of them is to play them, which raises your Nightmares even more. It really drives home how inescapable your nightmares have become.
Reaching Nightmare 6 by itself.
THEY ARE COMING THEY ARE COMING THEY ARE COMING
The Mr. Eaten storyline. It begins with a ravenous hunger in your stomach, and it only goes downhill from there...
Really, Seeking Mr Eaten's Name could have a Nightmare Fuel page all to its own. See the quotes page for some examples. I'll leave another one here:
Consume a Talkative Rattus Faber "Why are you looking at me like that? I knew a man used to look at me like that. Funny sort. Kept a candle in his - " Down in one "Wait no mmf mggl mmmmmfff!" The aftertaste is distinctly sewery, and you're still damnably hungry.
St Arthur's candle. The storylet where you get it implies you rend it out of your own body fat.
The Cave of the Nadir. Filled with a colour man was probably not meant to see, that rapidly robs you of your memories... and causes your skin and bone to grow over your eyes.
Some of the new Hallowmass storylets give you a peek into what could be Fallen London's endgame. In particular, this trope comes in with the Liberation of Night. You witness the Calendar Council setting off some kind of device that saps the light from everything in the Neath, along with killing (yes, killing) the Bazaar. Fire and chaos erupts all over London, exacerbated by the fact that no one can see - everything has ceased to shed any light. Civilians desperately flee for the exits back to the surface while looters and sorrow spiders take over. Your character is presented with a choice over who they must save;innocent strangers, your Constant Companion, or no one, if you're that ruthless (or if this was what you planned). In any case, there's a massive loss of life, an even more ghastly drop in the quality of life for everyone who survives, and the terrifying implication that the darkness will spread to the surface and beyond to all the universe. It certainly paints dealings with the Revolutionaries (and the aforementioned Nadir) in a different light.
Now you can see it coming...
You have minutely contributed to the Liberation of Night.
Even though the Recurring Dreams storylines increase your character's Nightmares attribute, they aren't actually that bad... Except for the ones that unlock at Nightmares 6, where the boundary between dreams and reality seems to break down.
Your character's slow descent into complete insanity at the cost of everything else in the Seeking Mr Eaten's name quest. Especially the bizarre and self-destructive actions you must take.
Polythreme. So far it's only appeared in the sidebar text, but it's terrifying. The city is enchanted with a magic that makes inanimate objects alive. This doesn't sound so bad until you realize the implications — as the sidebar gleefully points out, this includes things like candles.
Best illustrated by a card you can get if you play as a Clay Man:
"IN POLYTHREME THE BED I SLEPT ON WAS A SLAVE. THE ROOM WHERE I SLEPT WAS HACKED FROM SCREAMING STONE. THE WATER I DRANK BEGGED ME TO STOP. THEY PAID ME IN COIN THAT PLOTTED MY DOWNFALL. THE MEMORIES ARE TROUBLING. THIS PLACE IS BETTER."
The way to Polythreme is now open. From the hints dropped about it, one would expect a nightmarish hellscape, but it is actually...surprisingly upbeat. It comes off as more quirky and bizarre than legitimately terrifying. (Even the option to write a travelogue condemning it as a horrible place actually comes off as comical, because the only "horrors" you end up writing about are the milder ones.)
The third coil of the Labyrinth of Tigers. It's where the tigers keep humans locked up in horrible conditions, tucked away where few people will ever see them, and apparently held there indefinitely. It even gives your character nightmares just from walking around and hearing the screams!
"Get me out of here. I don't belong here. I'm only here because they think I'm possessed. Why am I here? I don't know how my family will eat now. I'm not possessed, I'm not possessed, I'm not possessed. I'm only here because I chased a cat! Where am I? For God's sake let me out. You there! I'm only in here because I organised a strike. Give me a mirror! Give me a mirror!"
It turns out they are not human, and are blackly dangerous monsters. You only find this out after you've had a number of chances to free many of them.
In the "Light Fingers" Ambition, The Orphanage, in its entirety. People are only sent here if their parents are dead (or murdered), and the entire place is a strange sort of asylum/hospital dedicated to creating and testing psychoses in the inmates, using the mind-affecting excretions of an eldritch monster.
The very concept of "snow" in an underground cavern.
It must be, in part at least, frozen water: when it melts, it refreezes as black ice. Tiny, desperate fish and insects can be seen frozen in the ice, no larger than a fingernail, eyes distorted with fear.