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.
Purported victims of incubi could have been the victims of sexual assault by a real person. Rapists may have attributed the rapes of sleeping women to demons in order to escape punishment. A friend or relative may have assaulted the victim in her sleep. The victims and, in some cases the clergy, may have found it easier to explain the attack as supernatural rather than confront the idea that the attack came from someone in a position of trust.
Incubi and succubi were said by some not to be different sexes but the same demons able to change their sex. A succubus would be able to sleep with a man and collect his sperm, and then transform into an incubus and use that seed on women. Their offspring were thought to be supernatural in many cases, even if the actual genetic material originally came from humans.
Though many tales claim that the incubus is bisexual, others indicate that it is strictly heterosexual and finds attacking a male victim either unpleasant or detrimental. There are also numerous stories involving the attempted exorcism of incubi or succubi who have taken refuge in, respectively, the bodies of men or women.
Incubi are sometimes said to be able to conceive children. The half-human offspring of such a union is sometimes referred to as a cambion. The most famous legend of such a case includes that of Merlin, the famous wizard from Arthurian legend.
According to the Malleus Maleficarum, exorcism is one of the five ways to overcome the attacks of Incubi, the others being Sacramental Confession, the Sign of the Cross (or recital of the Angelic Salutation), moving the afflicted to another location, and by excommunication of the attacking entity, "which is perhaps the same as exorcism." On the other hand, the Franciscan friar Ludovico Maria Sinistrari stated that incubi "do not obey exorcists, have no dread of exorcisms, show no reverence for holy things, at the approach of which they are not in the least overawed."
In Brazil and the rain forests of the Amazon Basin, 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. It is said to be responsible for disappearances and unwanted pregnancies, and it can never be seen by daylight, because it metamorphoses into kind of river dolphin during those hours. According to legend the boto always wears a hat to disguise the breathing hole at the top of its head.