Concept: a D&D campaign where every party member has been co-opted or replaced by some sort of hostile intelligence; e.g., the fighter has been possessed by a ghost, the wizard is being mind-controlled by her sapient magic ring, the rogue is actually a shapeshifting blob-monster who devoured the original and stole her form and memories, and so forth. Each of them is totally unaware of the others, and believes itself to be the only monster in a group of unwitting human adventurers.
Could you elaborate on those D&D settings? Maybe two or three sentences describing each? I always thought D&D was just set in Generic Fantasy Land #357