This is what we found on Wikipedia regarding the Incubus:

An Incubus (pl. Incubi) is a male demon who is described in various folklore as appearing in the dreams of female humans in order to seduce them. Repeated interactions between an incubus and a woman will lead to sexual activity, a bond forming between them, and ultimately sexual intercourse, as he requires vaginal ejaculation discharge to survive. The establishment and perpetuation of such a relationship enables the production of a hybrid child known as a cambion, but at the expense of the woman, whose mental and physical health will deteriorate rapidly, eventually resulting in her death if the incubus continues courting her for a protracted period.

In modern representations, an incubus is often depicted as a handsome man seducer or charming, rather than as demonic or frightening, to attract people instead of repulsing them. The female counterpart of the incubus is the succubus. Historically, folkloric belief in incubi was motivated by distressing nighttime phenomena, chiefly wet dreams and sleep paralysis.

In medieval Europe, union of an incubus or succubus and a human was supposed by some to result in the birth of beings half-demon and half-human. Legendary magician Merlin was said to have been fathered by an incubus. Walter Stephens writes in his book Demon Lovers that some traditions hold that repeated sexual activity with an incubus or succubus may result in the deterioration of health, an impaired mental state, or even death.