This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Call of Cthulhu
The Tabletop Game
All the typical Lovecraft stuff; cosmic horror, zombies, unfathomably ancient aliens etc... But then consider all the other mythos writers' creations that show up... And then add in some of the scenario writers' creations.
The monster that causes the problems in the scenario Crack'd and Crook'd Manse. A particularly graphic description plus the investigators realising just how big it is can call for a pretty rapid exit strategy from the characters. And the players.
The scenario The Colour of his Eyes from the San Francisco sourcebook. A man has a Colour Out Of Space (basically a form of sentient, malevolent light) trapped in his eyes, and he drains the energy from (and kills) anything he looks at.
The game actually tries to remain canon to Lovecraftian Mythos standards, as well. From the spell "Bind Lycanthrope" (which removes their humanity over the course of several days) to the fact that the Rat-things turn out to be, at least in some cases, created from willing cultists and unwilling murder victims, The Mythos is designed around the entire concept of Nightmare Fuel.
Which makes sense, once you know that H.P. Lovecraft had horrible nightmares that inspired a lot of his work...