Please don't list this on a work's page as a trope. Examples can go on the work's YMMV tab.
Nightmare Fuel: Blue Exorcist
Fujimoto Shiro as he's possessed by Satan, he gets a demonic appearance and starts disintegrating because Satan is too powerful for him. Cue a bleeding, horribly disfigured face. Then he rips his fingers off. The anime toned it down.
That depends on your definition of toned down. Sure, we don't actually see the Fingore happening; we just cut to Rin's expression of horror and hear them being torn off. For me, it was much worse.
Ghouls. They look like they crawled right out of something like Amnesia, being horrible, stitched-together, bleeding rotting things.
Rin as his face is literally punched off by Amaimon. Again, anime toned it down considerably.
The Impure King is a giant FUNGUS, not unlike the mostly harmless Coal tars often seen floating around. It spreads disease by making a gigantic "building" of its spores which grows over time and makes boils on the skin on anyone who touches them.
To say nothing of resident traitor Saburota Toudou regenerating while being shot by Yukio, always regenerating his mouth first to talk to Yukio about how "nice" it is to be a demon, and that Yukio is alreadylike a demon.
Let's be real here, that army of eyeless baby Yukios latching onto him as he tried to fight Satan's possession in episode 24 was pretty terrifying.
Recent chapters introduced us to a "ghost" in the girls' washroom. Aside from looking really, really horrible, there is what it actually is all the pent up frustrations of various teenage girls and what it says.
Chapter 43: the shapeshifter. Its true form looks like an Eldritch Abomination and can take the form of your worse fears if you look at it.
Chapter 48: Shima shows up to help Izumo and fights Takara, who appears to be the Illuminati spy. Then, at the very end, he stabs Izumo, and Takara calls him The Mole.
Chapter 51: Izumo's mother after all the experimentation done. She is bound to a wheelchair and rapidly deteriorating and the only thing she can say is requests for her daughter's help. Equal parts this trope and Tear Jerker.