Looking for a book with a bleak atmosphere that will give me a sense of wonder?

I'm looking for a very specific type of book and I don't know where to start looking. I would like to read something horror with a very bleak and hopeless atmosphere, heavily centered on atmosphere. I do not want something where I can expect anything from the world-building. I don't want zombies, vampires, dwarfs, wizards, ghosts… I want preferably something new or a very clever take on those typical creatures, transforming them into something original.

I don't necessarily want it to all make sense, I am not looking for something where every little detail is explained, kind of like Annihilation by Jeff Vandermeer: you start the book and know nothing, you finish the book and you arguably know less (I would be totally fine with a book that gives more explanations than Annihilation, though).

I am totally fine with gore and such, any genre is fine as long as there are some relevant horror elements.

Extra points if there's something like… humans transforming into something not easily identifiable that probably still holds, or used to hold, some amount of human consciousness.

Similar stuff would be Annihilation, Borne and The Strange Bird by Jeff Vandermeer, although these last two are relatively optimistic and happy reads. Lovecraft, obviously, although I don't really like short stories in general. In the videogame realm, I can think of Fear and Hunger, Dark Souls/Eldenring and Bloodborne. I know Berserk would probably scratch this itch but I want to read a book, not a manga/comic.