Nightmare Fuel: Possession. The Melange. The Beyond. Also, being a ghost. If you don't want to give up your mortal life, you spend an eternity in the Beyond, where 5 minutes feels like an eternity, all the other souls are screaming to get out, and when you do get out, the souls in the Beyond compel you to open up bodies for possession by souls from the Beyond.
The Beyond is a nightmarish void inhabited by the souls of the dead. The souls are reduced to powerless energy beings who can vaguely sense but never interact with the real world, and spend all their time digging through each others' memories because they can't do anything else. Incidentally, this same process causes them to perceive the passage of time at a slower rate, prolonging their already-eternal agony, and at least some have been there for tens of thousands of years.