Incubus aka Incubi, male shaped demons who seek out women are dangerous nightmare demons. Incubus and Succubus are both sex demons that can eventually kill their victims.
They are male demon nightmares, ghosts that sit on the sleepers chest and cause nightmares. Incubus are also lovers of witches, and the male counterpart to succubus, energy sucking spirits of sexual lusting.
Incubus demons take the form of males in order to have sexual intercourse with women. An Incubus may do this to father a child (Cambion), as in the legend of Merlin creating a half-demon offspring. Repeated intercourse with an incubus demon or succubus demon will end in the loss of health or even death of the victim.
Other Incubus stories include a feeling of something walking on the bed, finally kneeling on the victim's chest. The victim then experiences sleep paralysis and cannot move. Many victims claim that the demon resting on their chest makes it difficult to breathe.