Incubi and succubi are said to be nocturnal demons who are able to switch between male and female forms. A succubus: (plural: succubi) is a demon or creature of female form who has sexual intercourse with us in our dreams and fantasies, and an Incubus (plural: iccubi) is the male form. An example of an incubus is one who will pursue sexual relations with a woman in order to father a child, as in the legend of King Arthur and his master magician, Merlin.
They are sometimes called spirits, spooks, ghosts, fauns, fairies and vampires.
St. Augustine said in De Civitate Dei (“The City of God”); “There is also a very general rumor. Many have verified it by their own experience and trustworthy persons have corroborated the experience others told, that sylvans and fauns, commonly called incubi, have often made wicked assaults upon women.” These demons, and demon spawn were thought to be so real that thousands of people were burnt at the stake at the hands of Pope Innocent VIII in a Decretal Epistle, he commanded his Inquisitors to execute their Office on certain Heretical Women, who suffered themselves to be defiled by Daemons, both Succubi and Incubi.
The word incubus is derived from Late Latin incubo (a nightmare induced by such a demon); from incub(āre) (to lie upon). The earliest mentions of an incubus comes from Mesopotamia on the Sumerian King List in 2400 BC, where the hero Gilgamesh‘s father is listed as Lilu. It is written that Lilu visits and seduces women in their sleep, while Lilitu, a female demon, appears to men to have sex with them in their dreams. Many of the ancient philosophers thought they were Fauns; and Pliny calls the affection ludibria fauni.
The word nightmare derives its name from Mara, an evil spirit of the Old Germans and Scandinavians. They also called them, a spirit or goblin that rides on the chests of humans while they sleep, giving them bad dreams (or “nightmares“). The Ancient Greeks believed that nightmares resulted from the presence of a demon named Ephialtes. The ancient Assyrians, who called them limit. In Turkish culture, incubus is known as Karabasan. It is an evil being that descends upon some sleepers at night. These beings are thought to be spirits or jinns. In Brazil, the Boto is a combination of siren and incubus, a very charming and beautiful man who seduces young women and takes them into the river.
The a French novelist, Huysmans calls them “ecclesiastical microbes,” for the reason that monks and nuns are chiefly afflicted with them. The French jurist and political philosopher, Bodin mentions a priest who had commerce with one for more than forty years, and another priest who found a faithful mistress in a devil for half a century; they were both burnt alive (Demonomanie des Sorciers, p. 107). Luther was a firm believer in this intercourse (Ibid).(1)
Thomas Aquinas had written, “Still, if some are occasionally begotten from demons, it is not from the seed of such demons, nor from their assumed bodies, but from the seed of men, taken for the purpose; as when the demon assumes first the form of a woman, and afterwards of a man;
just so they take the seed of other things for other generating purposes.”
1. History of the Rise and Influence of the Spirit of Rationalism in …, Volume 1 By W. E. H. Lecky