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Nightmare Fuel: Geist: The Sin-Eaters
The Wretched are sin-eaters that have become Meat Puppets for their geists, unable to control their actions at least part of the time. Some are blessedly unaware of what their bodies are doing, like La Diabla Blanca, whose geist merely possesses her in her sleep. Others are not so lucky, and must watch from inside their own heads, unable to do anything as their geist uses their body for its own end.
Good news: you can come back from death if you're ever killed! Bad news: someone elsehas to die in your place, and you get to experience how they die just as you come back to life. And yes, you are aware that it's your fault, and yes, the experience can drive you mad.
The Stigmata Key. It makes its user bleed, it makes a sin-eater's enemies bleed, it makes the walls bleed... oh, and it can be used to command ghosts, who can in turn possess people.
A bit of Fridge Horror: geists used to be people. Now go back and look at the example geists, and realize that could be you someday.
The Phantasmal Caul lets a sin-eater literally become someone's worst nightmare made flesh. Given the image that accompanies this power in the core book, it's not surprising that merely looking upon a sin-eater using this power is a good way to go insane.