I keep circling back to cosmic horror, the stuff where the fear comes from realizing how insignificant and temporary your entire reality is. The most effective examples aren't just about monsters jumping out; they build a universe where the rules are wrong. I remember reading a short story years ago about a guy who found a crack in the wall of his apartment that seemed to breathe, and it expanded to reveal a sky full of non-Euclidian geometry and colors he couldn't name. The real chill came from the ending, where he just... accepted it. Went to work, made dinner, lived his life with this screaming cosmic truth in his living room. That quiet surrender to the incomprehensible stuck with me longer than any gory slasher scene ever could.
For novels, Thomas Ligotti’s 'The Conspiracy Against the Human Race' is more philosophy than narrative, but it genuinely messed up my sleep for a week. It argues that consciousness is a tragic evolutionary misstep. It’s not a story you "enjoy" in a conventional sense; it’s a bleak, persuasive thesis you absorb, and the horror is in how logically it dismantles any notion of inherent meaning or hope. I had to follow it with a stack of fluffy rom-coms to reset my brain.