An evil spirit or demon who was thought in medieval times to enter the bedchambers of sleeping women and to copulate with them as they slept.

Such a demon supposedly impregnated the daughter of the King of South Wales, who later gave birth to Merlin.

A "frozen demon" named Incubus appears in Thelwall’s The Fairy of the Lake as the servant of Hela, Queen of Hell. A braggart in life, he fled when confronted with his first battle and died of apprehension. The Saxon Rowena called upon his help in her plan to seduce Arthur.

The female shape, who seduces men, is called succuba.