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Nightmare Fuel: Earthdawn
Horrors. They are a group of entities from the depths of astral space. Every few thousand years, when the magical field permeating the Earth is at its strongest, they return to our world to feed. On everything. If they don't eat the mountainside (and you with it), they torture you mentally and physically for sustenance and fun. And the threat of death, Mind Rape and actual rape are really the least you have to worry about in the long run with their guys; they'll gladly have you as a self-aware puppet (forever) with a bad case of Body Horror who will kill and/or otherwise do unspeakable things to your friends and loved ones in order to feed your new master. And the torment it puts you through is dessert. They might even corrupt your psyche and everlasting soul, and turn you into one of them. And don't study up on them too much to find out their (few) weaknesses, it can get their attention.
And they exist in Shadowrun. See the "Harlequin's Back" module.
The Blood Elves. To defend themselves from the Horrors, they used a powerful magical ritual to cover everything in their forest home in thorns. Everything. Including themselves. The race survived by virtue of living in such pain that the Horrors were no longer interested in tormenting them, for the most part.
What about some of the spells in Earthdawn? Just look up "Skin Shift" or "Onion Blood". Or better yet, don't.
The 'Horrors' book. According to the dragon legends a Horror called Verjigorm created the first Dragon by accident while sprouting all kind of Horrors. Now he's hunting them and corrupting even while the magic level is too low for other Horrors. There is also Nebis, whose name is a nice cause of Speak of the Devil, and who rebuilds himself from the bodies of those who have slain his physical body. Or Ristul, who isn't an actual entity, but the corruption itself. And those aren't the most interesting ones. If you look up the Earthdawn bestiary, you'll see that even cows and rabbits can be nightmarish.