Here are Stan Marsh, Kyle Broflovski, Kenny McCormick and Eric Cartman in different horror AUs.

Monster AU - Stan is a vampire, Kyle is a warlock, Cartman is a ghost in chains and Kenny is Frankenstein's monster and almost everyone else is normal (except for Craig, Clyde, Jimmy, Tolkien and Tweek who are also monsters). While the world fears monsters like them, these four wanted to make friends with other kids (even though Cartman is more interested in scaring).

Ghost AU - Stan, Kyle, Cartman and Kenny have dead for years, but their ghosts still haunt in streets of South Park, unable to rest in peace. While Stan, Kyle and Kenny have no intension on scaring anyone, Cartman actually enjoys scaring people.

The Boys in Dark Deception (South Park/Dark Deception AU) - Stan, Kyle, Cartman and Kenny were brutally murdered, but their souls had been reincarnated as ghastly, undead, monstrous versions of themselves as Malak's minions. Cartman and Kenny can also turn into nightmarish versions of the Coon and Princess Kenny respectively. Also, Butters, Craig, Clyde, Jimmy, Tolkien and Tweek are the ones being haunted while Butters searches for the Soul Shards.

South Park: Chains (Ghost!Cartman AU) - Eric Cartman had died at the age of 10, but he comes back from beyond the grave as a ghost, bound in chains as punishment for the crimes he committed in life and unable to rest in peace, and still haunts his friends, including Stan, Kyle and Kenny, who have grown into adults and are used to his ghostly antics.

South Park (c) Trey Parker, Matt Stone, South Park Studios, MTV Entertainment Studios and Comedy Partners (Comedy Central)
Dark Deception (c) Glowstick Entertainment
Idea and artwork (c) me