Nightmare (n) - A bad dream. Night is an Old English word. Mare is also Old English word for an evil spirit. Combined, the words described the incubi and succubi. Respectively male and female demons that have sexual intercourse with sleeping people, feeding off the sleeper's soul.
Incubus and Succubus (n) - Both from Latin. Incubus means one who lies on top of a sleeper. Succubus means one who lies underneath a sleeper. These demons are staples of World of Warcraft, second-rate horror movies, and fundamentalist Christian sects trying to scare believers away from sex.