In medieval European folklore, the incubus is a male demon (or evil spirit) who visits women in their sleep to lie with them in ghostly sexual intercourse. The woman who falls victim to an incubus will not awaken, although may experience it in a dream. Should she get pregnant the child will grow inside her as any normal child, except that it will possess supernatural capabilities. Usually the child grows into a person of evil intent or a powerful wizard. Legend has it that the magician Merlin was the result of the union of an incubus and a nun. A succubus is the female variety, and she concentrates herself on men. According to one legend, the incubus and the succubus were fallen angels.
The word incubus is Latin for "nightmare".