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.
Unfortunate Implications: "Ethnic Streaming" - At first, Earth's government sponsored extrasolar colonization on a cosmopolitan basis; immigration was controlled so numerous ethnic groups were represented equally. This experiment came to a crashing end when these colonies erupted into religious conflicts. Nyvan, the worst case, had nonstop holy wars right up until the Possessed showed up and conquered the whole planet with superpowers. Afterwards, colonies were sponsored by specific nations and corporations, which limited colonization to specific ethnic groups, creating Planets of Hats. These were much more successful, and by the time the story begins, planets are specifically stated to be Christian, Islamic, Jewish, Hindi, etc. and it's commonly accepted that only an idiot would expect multiple ethnic groups to share a planet peacefully. A similar idea would be "separateness" AKA "Apartheid".