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. 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.
Medieval legend claims that demons, both male and female, sexually prey on human beings. The male demon is known as an incubus and the female is the succubus. They generally prey upon the victim when they are sleeping, though it has been reported that females have been attacked while fully lucid.
The incubus is sometimes confused with the legendary "Old Hag" syndrome. The Old Hag episode, however, is usually restricted to an unpleasant feeling of great pressure on the chest and not a ghostly sexual encounter. Origins
A number of secular explanations have been offered for the origin of the incubus legends. They involve the medieval preoccupation with sin, especially sexual sins of women. Victims may have been experiencing waking dreams or sleep paralysis. Also, nocturnal arousal, orgasm or nocturnal emission could be explained by the idea of creatures causing an otherwise guilt-producing and self-conscious behavior. Alternately, the influence of incubi could also have been invoked to explain otherwise "unexplainable" pregnancies out of wedlock.