This is based on opinion. Please don't list it on a work's trope example list.
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.
Zombie Chimeras are lurching, snarling, slavering masses of Body Horror—huge, twisted abominations that have way, way too many eyes, mouths, ears, limbs... they make ghouls look like swimsuit models by comparison. They're the horrific results of immortality/regenerative experimentation derived from studying Izumo's mother, and they used to be people. Some of them can still think and talk, enough to react with surprise to Shiemi screaming, or utter a desperate plea to be allowed to go home...
Yukio in chapters 71 and 72 of the manga. He's trying to get his Eye power to work, but as it apparently kicks in when he's feeling an intense emotion, he repeatedly makes attempts on his life, like trying to drown himself to experience true fear. It doesn't work. Chapter 72 ends with him jumping off a building and utterly terrified,and as he falls, thinking that he doesn't want to die.
It then gets even worse in ch73: upon finding out that a) he survived and b) his power only briefly flared up and still isn't stable he puts a fucking gun to his head and demands that the "monster" comes out. Holy. Shit.
Chapter 76 ends with the strong implication that Hachiro is going to force Shura and Yukio to conceive a child through Mind Manipulation against their wills. Keep in mind that he's underage, they've essentially had the same adoptive father, and Shura realizes that he may have done this to the other women who came after her ancestor. Brrr!
Chapter 77 starts with the above implication nearly happening. If Rin hadn't shown up, it would have gotten much worse. And even so what Yukio suggests and says is incredibly freaky even if he's acting, he didn't have to shoot Rin - he's too calm about it too! Followed by Hachirolosing it and going berserk. Did we mention that Hachiro is immortal and the previous Paladin was ultimately powerless against him? And now he's planning on killing them all...and Yukio's "successful" plan also weakened Shura.