In the Mage: The Ascension sourcebook Technomancer's Toybox, one of the items on display is a VCR developed by the Technocracy that makes the viewer believe whatever he sees on the video. It was originally developed to facilitate Laser-Guided Amnesia for the group's The Men in Black enforcers. Then a group of them decided to experiment on some guy by making him watch his kid's Barney the Dinosaur and The Smurfs videos. Now the poor bastard spends all his time in front of his TV waiting for his little friends to come out and play with him again...
Vicissitude from Vampire: The Masquerade: Body Horror as an ability. Why use handcuffs when you can meld his wrists together? A straightforward use is to suffocate someone by sealing his mouth and nostrils. A more spectacular approach is to merge dozens of people together into a living siege engine, a huge mass of flesh, pain, and madness. One of the masters of the craft remade himself into an androgynous beauty with lines across his skin that are actually hundreds of tiny, functional mouths that whisper along with him as he speaks.
In relation to that: Cathedral of Flesh. Which is Exactly What It Says on the Tin. It's hard to tell what's more nausea-inducing, the fact that it's a flesh-church where the pieces are still alive or the fact that the Tzimisce who's talking about it is doing so with such loving nostalgia.
Ken Meyer's illustrations for the original Players Guide to the Sabbat. Blood parties. Chainsaw artists. Creepy chicks emerging from mirrors. Evil jesters with bloody-mouthed kiddies? almost every image in this book embodies this trope to maximum effect.
The Vampire: The Dark Ages splatbook Clanbook: Baali is terrifying. At the start, a corrupt abbot has graphic sex with an altar boy, who floods the abbot's body with vermin. More and more things crawl into the abbot, causing a mix of horrible pain and ecstasy as he is in the middle of intercourse until the abbot passes out. Over the next couple weeks the abbot is tortured, fed nothing but maggots, flies, and other insects, feels insects crawling on him and inside until they eat his way out. We learn that the "youth" is Really 700 Years Old and a vampire. Further into the book we learn that there are a number of slumbering Eldritch Abominations who are only kept from waking up and destroying the world by a steady stream of atrocities performed by vampires and mortals. This means that gang rapes, genocide, murder, and torture are all necessary things. Peace and happiness in the world leads to the death of all.
There is a damn good reason Clanbook: Baali is a Black Dog release.
Wraith The Oblivion was made of this trope. The entire game was about psychological horror, screwed up internal conflict, evil shadowy voices constantly whispering your worst fears to you, an all-consuming eldritch power wanting to snack on your soul like a tic tac and make you into one of its soulless faceless soldiers, and, oh yeah, did we mention that you're dead when the game starts, and that you're trapped in a dark, dreary, hopeless afterworld populated by other tortured souls? This is really just scratching the surface mind you. Wraith isn't just nightmare fuel, it's a frigging Nightmare Fuel perpetual motion machine that runs off of nightmare fuel and then produces more nightmare fuel than was put it to get it started!Changeling: The Lost is the only White Wolf game that might have been more grim and depressing. Might!
And it gets worse. Wraith had an entire book dedicated to the Holocaust's effect on the world of the dead. The scariest parts of the book aren't related to the game, though; it's the passages describing what actually happened in real-life.
Werewolf: The Apocalypse has Fomori, people infested by Banes (spirits corrupted by the Wyrm). Body Horror aplenty, twisting the poor soul into something horrifying. Once infested, it is all but impossible to separate the Bane from the person's soul (especially without killing them), meaning that when they finally die they probably get to spend the rest of eternity in Malfeas, where they will be tormented and tortured for the rest of time. And the worst part about them? Just how easy it is to become one; if you're in the wrong place at the wrong time, you can have one shoved down your throat. If your soul suffered damage through traumatic events, a bane can sneak in those cracks and corrupt you from the inside out. And most horrifying of all, an entire umbrella organization, Pentex, is dedicated to spreading these things through their subsidiaries. Get IVF through one? The baby is born a Fomori. Buy Tabletop Games from Black Dog? Have fun as they turn you into a monster. Want to stop for some fast food? Enjoy the unwanted passengers. And once they're in, you're doomed to a Fate Worse Than Death.
The Midnight Circus. No PC is safe from it, no matter which game you are playing. Approach at your own risk.