This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Geist: The Sin-Eaters
The mere presence of a Sin-Eater reinforces the surrounding ghosts whether they like it or not, effectively allowing them to manifest more easily; a Sin-Eater can single-handedly plague an area with ghost activity and apparitions just by being here. Which can become pretty bad if said ghosts are vengeful or malevolent...
The Hunter supplement Mortal Remains reveals Slashers are terrified by Sin-Eaters for this very reason; they don't expect their victims to come back for revenge, which is exactly what can happen when a Sin-Eater shows up and allows the pissed off ghosts of all the people they killed to come back. You read that right; the Bound are in-universe nightmare fuel forserial killers.
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.
You also have to take in account what this means for a Sin-Eater's enemies. Sure, for the Sin-Eater, dying too many times is a bad thing because it gradually makes him lose his control over his Geist... but that doesn't make it a good new for his opponents. A Bound who runs out of Synergy and becomes a Wretched won't actually suffer Final Death, he will keep coming back while growing more and more insane, meaning he is unlikely to stop fighting. His opponents just can't hope to actually kill him, just delay him for around a night. And someone dies each time he comes back.
Some of the powers granted to Sin-Eaters by the various Keys and Manifestations are truly horrifying:
The Stigmata Key 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.
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.
The Tear-Stained Rage allows you to fill your opponent's lungs with water, drowning them on the spot.
The Pyre-Flame Rage allows you to set people on ghostly fire. At lower levels, the flames are purely psychosomatic, meaning they don't truly harm (though they still are painful), but at higher levels, they become quite real and often burn the victim entirely to ash.
A bit of Fridge Horror: Geists used to be people. As in, they started out as humans, became regular ghosts, and then turned into Geists by gradually trading their Anchors for various aspects of Death. Now, go back and look at the example geists, see how utterly inhuman some of them look, and realize that could be you someday.
Most Ceremonies practiced by Sin-Eaters are used to help ghosts, communicate with them and move to the afterlife. Then, you got "Quicken the Dead", which allows you to trap them in corpses and make them obey you, essentially creating yourself a personal group of zombies.
In general, Ceremonies can have pretty fucked up effects when in the wrong hands. Book of the Dead's opening fiction features a Yandere Sin-Eater who kills his ex in the Underworld, slays her Geist, then tries to forcefully bind her ghost to an Anchor he created so he can bring her back on Earth, have her possess a mortal woman and keep her forever close to him.
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 include 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.
Avernian Gates, which basically are portals to the Underworld. Now the idea of portails that could open at any moment to lure you in the realm of the dead or unleash insane centuries-old ghosts is pretty scary in itself, but then Book of the Dead provides us with a few sample famous Gates, which show us how truly dangerous these places can be:
Another has the ability to attract Death Seekers to then warp them into vicious, ravenous shades who then wander around next to it. Unfortunately, that Gate happens to be in the Aokigahara Forest, a place famous in Japan as a spot regularly used for suicides.
The Hagia Sophia has one hidden below it, where lied many treasures and powerful artifacts; unfortunately, it also happens to be the lair of a gigantic, arachnid monster, who uses all these treasures as baits to lure Sin-Eaters to it so it can devour their Geists.