Incubi are a type of lesser demon left over when the Prim receded and have adapted to live in the world created by Gan.
They are all male, the female version being Succubi, and feed off of sleeping humans by mating with them while they sleep, drawing off a little of the person's 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.