An incubus (plural incubi) is a demon in male form supposed to lie upon sleepers, especially women, in order to have sexual intercourse with them, according to a number of mythological and legendary traditions. Its female counterpart is the succubus. An incubus may pursue sexual relations with a woman in order to father a child, as in the legend of Merlin, and some sources indicate that it may be identified by its unnaturally cold penis. Religious tradition holds that repeated intercourse with an incubus or succubus may result in the deterioration of health, or even death.
A succubus (plural succubi) is a demon who takes the form of a beautiful woman to seduce men, especially monks, in dreams to have sexual intercourse, according to the medieval European legend. Their male counterpart is the incubus. They draw energy from the men to sustain themselves, often until the point of exhaustion or death of the victim. One such story relates to a man in the town of Coblenz, who has been bewitched by a succubus, with whom he is forced to repeatedly fornicate, whilst in the presence of his wife. The story goes on to say that “after an incredible number of such bouts, the poor man at last sinks to the floor utterly exhausted.” From mythology and fantasy, Lilith and the Lilin (Jewish) and Lilitu (Sumerian) are in redactive Christian fables (folktales not part of official Christian theology), considered succubi.