Incubus

An incubus (nominal form constructed from the Latin verb, incubo, incubare, or "to lie upon") 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, as in the legend of Merlin. Religious tradition holds that repeated intercourse with an incubus or succubus may result in the deterioration of health, or even death.

Source: http://en.wikipedia.org/wiki/Incubus