Even if we forget about all the Cthulhu-esque horrors from beyond reality, we have such interesting cases as Tomb Beasts, lizards who have the curious and specific habit of eating their prey only while it still lives and never, ever eating any dead flesh. As such, they will carefully vivisect slices of flesh one at a time, so that their prey does not die until as much as possible is consumed.
On the bright side, there are hundreds of people with powers far beyond any mortal. Except they're all going slowly insane, and half of the most powerful ones are seeking to destroy/conquer Creation for their dead/imprisoned (respectively) masters.
And several of the ones without said masters are still potentially looking to do so. And in the case of conquest, they have a divine right to do so.
A special note for the Abyssal Exalts: they've been created with the explicit purpose of destroying all of existence, and their powers have been specifically tailored to maximize their destructive potential. And if they choose not to carry out their ordained purpose, they have a high chance of accidentally killing their loved ones.
The Infernal book has to lead the charge, though, with poor Lillun. Squick, certainly, but reading her story gave me the need to put the book down and go watch some cartoons for a while. With the lights on.
The Infernals cover◊. That thing in the bottom right is one guy's right arm. You can go curl up in a corner now.
The Ebon Dragon is a walking, talking purveyor of this trope. Remember why that Stephen King short story "The Jaunt" is so horrible? The Ebon Dragon can do that to anyone he kills. Have fun spending eternity in a black oubliette where you will only experience the passage of time, from which you can't be released. No more fun for you, ever.
The Compass of Celestial Directions: Malfeas. The whole book, but especially the descriptions of specific locations in Malfeas, such as the Screaming Cathedral.
Their nickname, the stomach bottle bugs, comes from the tendency of some demonologists to store sesseljae inside their own stomachs, as a preventative measure against poison and disease. While practical, this understandably creeps out many people.
Any description as to how the Exalted transformed Theion into Malfeas is terrifying on a cosmic level. According to the books, they castrated him and then turned him inside out to make him into what he is. Thus, it's understandable why he's essentially the embodiment of every anger trope in existence.