This is what we found on Wikipedia regarding the Incubus:

An Incubus (pl.  Incubi) is a male demon in human form in folklore that seeks to have sexual intercourse with sleeping women; the corresponding spirit in female form is called a succubus. Parallels exist in many cultures.

In medieval Europe, union with an incubus was supposed by some to result in the birth of witches, demons, and deformed human offspring. 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.