An Incubus is a demon in male form who, according to a number of mythological and legendary traditions, lies upon sleepers, especially women, in order to have sexual intercourse with them. Its female counterpart is the succubus.
An incubus may pursue sexual relations with a woman in order to father a child. 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, the spirits' offspring were often thought of as supernatural. The half-human offspring is referred to as a cambion.