Succubi are a type of lesser demon left over when the Prim receded and have adapted to live in the worlds created by Gan.
They are all female, the male version being Incubi, and feed off of sleeping humans by mating with them while they sleep, drawing off a little of the persons khef as they do so.
There are also more predatory ones which feed off waking humans. These sexual vampires, as they are known, transform into beautiful looking people when they come across lonely humans and then seduce them. They will then suck the human dry unless they can break free of their grasp.