A Succubus is a demon from folklore that was said to haunt the dreams of sleeping men, mating with them to produce various offspring or to simply drain them of life-force - the Succubus was almost always depicted as female and unlike the Night Hag was normally an attractive and seductive spirit.
The male version of a Succubus is known as an Incubus and delivers the same role, though Incubi are a little more violent in nature - both are deadly creatures of the night and were born from medieval legends spawned from a period of history when the idea of chastity was sacred.
The Succubus has become a staple of fiction ever since and has always retained her basic role as a seductress, though her exact nature and role has altered in every retelling.
The Four Most Powerful Succubi are the Four Queens of Hell; Lilith, Naamah, Agrat, and Eisheth. These four also are the wives of Terrantos in the Beyond Light and Darkness Series and never were mentioned In the original Story.