The tone of AUs.
AUs that, as opposed to Comedic AUs, actually hold a serious story and such. Just because an AU has a serious tone does not mean that it must be serious all the time, just most of the time.
AUs that are mostly comedic in nature. Bear in mind that a Parody can be either dark or comedic so Parodies and Comedic AUs aren't necessarily the same thing. Nevertheless, they can also be both.
AUs that feature horrible, evil, and/or murderous Monsters.
Warning: Some of these AUs contain themes that may be disturbing to some viewers.
AUs with the goal of creating something scary, insane, or disturbing. Some AUs that follow the genocide route are also found here, but not all are geno-route-only. Some possible themes include suicide, drug use, alcohol use, depression, death, gore, insanity, disfigurement, pain, etc. Horror can also include Terror, and effects on psychology. This could include things like dystopias and the like.
It is suggested you read the short bios of these before googling them!