Two days late, whee! But this is THE horror fic for me, so I had to rec it.
Fandom: Harry Potter/Anita Blake crossover
Author: Mhalachai (mhalachaiswords)
Comments: A late-night run-in with werewolves in the woods outside St. Louis dumps Harry Potter into a whole new world of trouble. Now Anita Blake has to deal with a new charge as well as Death-Eaters come to town.
I'm not reccing this for horror because of the vampires, or the werewolves, or the other assorted things that go bump in the night. I'm reccing this for horror because of the terrible hollow feeling that I got inside when reading it. Some ways into the fic, Anita starts undergoing some serious, serious mindfuckery, and experiencing the inside of her skull as Mhalachai writes it was very disturbing. She's hurting, in agony, and terribly isolated, and terribly desperate that no one must ever find out, and to me it was a disturbingly effective trip down a road of psychological horror.
I don't guarantee that everyone will share this experience; this fic may press my psychological horror buttons but not necessarily other people's. But for what it's worth, here's a fic that truly managed to give me a scary hollow feeling inside.