The main enemy is a race of Lovecraftian creatures. Let's call them The Knowed Ones. They always appear 12 feet tall; no matter how low the hall or room is, the world will seem to bend around them. The closer they get to you, you begin to experience dizziness and short-term amnesia. Too much exposure to them can cause total amnesia, insanity, coma, and eventually death, whereupon the victim will be reanimated as a Knowed One. A special few have trained themselves to be immune to some of the Knowed One's effects, and dedicate themselves to fighting them.
As you can tell, I've had this idea for a while.