An Incubus is a male demon that shares a similiar role to his female counterpart, the Succubus, however unlike the seductive Succubus the Incubus was a more forceful spirit and was believed to invade the dreams of women at night and rape them, often producing evil offspring as a result.
Incubus attacks involved phenomena such as a shortness of breath, paralysis and nightmarish illusions - which may of been caused by the very real condition known as Sleep Paralysis: however in the superstitious and highly devout minds of the medieval world such a thing could only be registered as an unholy defilement of one's most vulnerable moments.
Another sad theory is that many "Incubus" attacks were created in order to try and explain unwanted pregnancy or even the crime of rape itself - after all it was far easier to blame such things on invisible, bestial monsters than on the very real human element.
However to those who believe in the hidden world of ghosts, demons and otherworldly beings the Incubus is simply an evil spirit and even in the modern world their are cases of people that claim to have been visited by Incubi or similiar entities - one of the most famous modern examples was in the film "The Entity".
Like many other "night spirits" the Incubus can also be explained thanks to the discovery of the medical condition known as Sleep Paralysis.