In medieval legend, a succubus was a demon which takes the form of a female to seduce men in dreams to have sexual intercourse. They draw energy from the men to sustain themselves, often until the point of exhaustion or death of the victim.
Fox Mulder believed Walter Skinner may have been visited by a succubus that resulted in the murder of a prostitute with whom Skinner was sleeping with. Mulder said the succubus often took on the ghostly image of an old woman, which Skinner had been witnessing. (TXF: "Avatar")