incubus

A demonic spirit of the night who sits on the chest of a sleeping person, causing nightmares. The incubus was said to seek carnal intercourse with women in their sleep. In the Middle Ages, the existence of incubi was recognized by ecclesiastical and civil law.

The female version of the incubus is the succubus.

The name comes from Latin incubo, "nightmare," and incubāre, "to lie upon."