This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Geist: The Sin-Eaters
The Wretched are sin-eaters that have become Meat Puppets for their geists, surrendering control to their geists some or all 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 wears them like a skin suit.
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. Even if it doesn't screw you up, a bit more of your control over the geist crumbles every time... and your potential control withers.
The Stigmata Key, which grants power over blood and ghosts. 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.
When combined with the Caul, it can also be used to detach your arm and control it remotely. Or similarly control a pool of your own blood remotely and see through it. Or spawn an Homunculus from your very blood, essentially making a blood golem. Sweet dreams.
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.
Phantamsal Marionette allows the user to form a plasm-copy of their Geist's body and plant their mind inside of it. Higher uses allow the user to increase the power of this body, making it a more effective method of reverse-possession - the player also gains bonuses to their Intimidation skill, and considering what a Geist can look like...well, it's quite disturbing to think of.
Fetters are mementos that are powered by ghosts trapped within them. They are still quite conscious, but live only as tools for sin-eaters that want to make use of their numina. Sometimes it's a fitting punishment (like the movie camera holding a Snuff Film maker, the rifle holding a mass kidnapper, the spray can holding a ghost who never got the message about Disproportionate Retribution...), but many Sin-Eaters don't care about whether the ghost deserves it.
The Underworld itself. Best-described as an Eldritch Location version of Catacombs, this never-ending, underground-like world is where ghosts go when they move to the Afterlife, presumably so they can reach a better place beyond (though no one knows for sure about that part). Unlike in the physical world however, this place actually allows them to evolve and grow in power, meaning ghosts and shades in this place will be much more powerful and intelligent, and all the more dangerous if they are malevolent or insane. Other inhabitants included Geists who still have yet to find a host and Kerberoi, some places are just plain bizarre, enough to cause a Sin-Eater to lose his mind if she isn't careful, and have strange, undesirable side effects. Oh, and if you spend too many night inside and then get killed, don't count on your Geist to bring you back; you will be stuck here until someone else comes to rescue you.