An incubus (plural: incubi) is the male equivalent of a succubus, one of the lesser daedra descended from Suspira and her countless sexual partners. Incubi feed and reproduce by mating with humans and other humanoid beings, drawing on the life force of their partners even as they fill their wombs with their magically potent seed.
A human woman inseminated by an incubus will give birth to an apparently normal human child that transforms into an incubus or succubus upon reaching adolescence.1 Impregnation is virtually guaranteed with these creatures, unless magical contraception is used.2 This is the primary (and perhaps only) way that incubi reproduce; it is believed that incubi and succubae are unable to draw energy from each other, and thus will mate only with mortals.
An incubus in its natural form resembles a very attactive human with reddish skin, red or yellow eyes, black hair, a prehensile pointed tail, batlike wings, and short, stubby horns protruding from the upper corners of the forehead.3 In this form they are sometimes mistaken for tieflings.4 However, these daedra are consummate shapeshifters and can take on any humanoid form, maintaining it indefinitely; because of this, many of them masquerade as human while on the Material Plane. Succubae can have sex in male form and incubi in female form; they are only fertile when in their native genders, but they can draw energy from their partners regardless of the genders of the participants.
Incubi, Succubae, and Mortal Souls
Because incubi and succubae are always half-human (or half-mortal, anyway), they have a mortal soul in addition to their daedric Essence. This means that incubi and succubae have two things that other lesser daedra do not have: the capacity for free will, and the ability to develop a conscience. It is rare for their human sense of morality to survive the transition from childhood into their mature daedric form, but it is possible for an incubus or succubus to rekindle its lost humanity, particularly if it forms a bond with a mortal who can "remind" the daedra's soul of its true nature.
It is also possible for a teenage incubus or succubus to be purged of most of its daedric essence, if its true nature can be discovered before it completes its metamorphosis into an adult daedra. The ritual to do this is very difficult, not to mention painful for the subject — but if it is successful, the subject will become a tiefling, with only minor signs of his/her daedra heritage, and his/her human personality will remain intact. There may not be any outward signs if the ritual fails, particularly if the daedra is clever enough to disguise itself as a tiefling and thus appear "cured".