The Incubus and Succubus are two different types of spirits, who are known as “sex demons” or “night demons” because they are believed to engage in sexual activity with with men and women during the dream state. The Incubus is believed to be male, while the Succubus is female.
According to Wikipedia:
Incubi were thought to be demons who had sexual relations with women, sometimes producing a child by the woman. Succubi, by contrast, were demons thought to have intercourse with men. Debate about the demons began early in the Christian tradition. St. Augustine touched on the topic in De Civitate Dei (“The City of God”). There were too many attacks by incubi to deny them. He stated, “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.” Questions about the reproductive capabilities of the demons continued. Eight hundred years later, Thomas Aquinas lent himself to the ongoing discussion, stating, “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.” This view was also shared by King James and in his dissertation titled Dæmonologie he refutes the possibility for angelic entities to reproduce and instead offered a suggestion that a devil would carry out two methods of impregnating women: the first, to steal the sperm out of a dead man and deliver it into a woman. If a demon could extract the semen quickly, the transportation of the substance could not be instantly transported to a female host, causing it to go cold. This explains his view that Succubae and Incubi were the same demonic entity only to be described differently based on the sexes being conversed with. Being abused in such a way caused women at nunneries to be burned if they were found pregnant. The second method was the idea that a dead body could be possessed by a devil, causing it to rise and have sexual relations with others. This is similar to depictions of revenants or vampires and a spirit taking deceased corpse to cause some mischief. It became generally accepted that incubi and succubi were the same demon, able to switch between male and female forms. 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. Even though sperm and egg came from humans originally, the spirits’ offspring were often thought of as supernatural.
Some sources indicate that it may be identified by its unnaturally large or cold penis. Though many tales claim that the incubus is bisexual, others indicate that it is strictlyheterosexual and finds attacking a male victim either unpleasant or detrimental.
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. An incubus may pursue sexual relations with a woman in order to father a child, as in the legend of Merlin.
According to the Malleus Maleficarum, exorcism is one of the five ways to overcome the attacks of incubi, the others being SacramentalConfession, the Sign of the Cross (or recital of theAngelic 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, theFranciscan friarLudovico 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.”