Incubus' According to Jewish and Christians by Jennifer Scelsi

 The Incubus According to the Jewish Faith

The existence and activities of incubi are acknowledged in Jewish demonology in relation to a Midrashic legend of Adam's siring demonic offspring. According the kabalistic text the zohar, these unions are continued by men who who unknowingly cohabit with spirits in their sleep. The hybrid human-demonic children have a demonic nature and rank high in the echelons of demons, occupying positions of authority and ruler ship. Thus demons value intercourse with humans.

The Incubus in the early Christian church

According to some of the fathers of the church, among them St.Justin Martyr, Clement of Alexandria, and Tertullian, as well as the Jewish historian Josephus and some Platonist philosophers. the incubi were the Sons of God, angels who fell from heaven because they copulated with women.
Their offspring were giants, the Nephilim, who could not have been the progeny of human men and women.

St.Augustine included among incubi the pagan demi-gods sylvans and fauns, who were exceptionally lascivious and often injured women in their lust, he said.