From Wikepedia
"In Western medieval legend, an incubus (plural incubi; from Latin incubare, "to lie upon") is a demon in male form supposed to lie upon sleepers, especially on women in order to have sexual intercourse with them. The Incubus drains energy from the woman it performs sexual intercourse upon in order to sustain itself, and in most cases either kills the victim or leaves the victim alive but in very weak or fragile condition. A female version was called a succubus. There are several possible explanations for the incubus legends:
Women who were pregnant but not married would often accuse an incubus, or when explanation were needed to obscure evidence of extra-marital affairs.
Waking dreams or nightmares.
They form part of the well-attested Medieval preoccupation with sin, especially sexual sins of women.
Actual rapes of sleeping women were attributed to demons by rapists in order to escape punishment.
The feeling of smothering while sleeping is known since antiquity as nightmare. The modern term for this state is sleep paralysis.
Because of the weight given to sexual sin in the Middle Ages, nocturnal arousal, orgasm or nocturnal emission were explained away by the legends of creatures causing an otherwise guilt-producing and self-conscious behavior. Thus people could say they were not to blame for it; it was obviously outside of their control: they were a victim.
Young women/men being sexually assaulted in their sleep by a known attacker such as a friend or family member, although not common, has been reported and may explain some nocturnal attacks. The victims may find it easier to explain the attack as supernatural rather than confront the idea that the attack came from someone that is trusted in the family. See incest or "situational molesters".
During the witchhunts, alleged intercourse with demons or with Satan was one of the purported sins for which women were killed.
Sometimes incubi were said to conceive children with the women whom they raped; the most famous legend of such a case includes that of Merlin, the famous wizard from Arthurian legend.
In some legends, incubi and succubi were said not to be different genders of the same demonic "species," but the same demon able to change their sex; the idea being that a succubus would be able to sleep with a man and collect his semen, and then transform into an incubus and use that seed on women. Nevertheless their offspring were thought to be supernatural in many cases, even if the actual genetic material originally came from humans. This particular idea was explored in the Stephen King series The Dark Tower. In the modern era, succubi have featured in roleplaying games as tantalizing imagery for marketing. The theme of the incubi has continued into modern times as well, with stories of unseen paranormal beings which rape female victims such as Invisible Masters and the creature in the movie The Entity"
