The incubi are powerful, fleshless beings that have been around for millenia, possibly since the beginning of time. They can be imprisoned but cannot be destroyed. They are also known as devils of influence, powers, forces, thrones, dominations, gin, daimon, and dark spirits.
The earliest forms of sorcery were built upon their power. Incubi taught men sorcery and were paid by their students with what men had but incubi lacked—"flesh, blood, the service of minds and souls that can grow and change."
Back in those days, people sometimes confused incubi with imps and faeren .
The old kings "struggled against them," but because incubi cannot be destroyed, paupers, greens, and prophets bound them in stones, glass, and gems and sealed them with "names of power." One of these stones was called the Blackstar, and it contained thousands of incubi. 
Considering the descriptions of the incubi given in the books and some of their alternate names and also that N.D. Wilson is a Christian, it is likely that the incubi are demons.
- "There are evils older than Endor....as old as the stars....The old stories call them many things—devils of influence, powers, forces, thrones, dominations, gin, daimon, and incubi. They were said to be fleshless—dark spirits—and many even muddled them with imps and faren. The first sorceries were built on their powers. Men made alliance for the dark strengths and paid those tutors with what they lacked—flesh, blood, the service of minds and souls that can grow and change. The old kings struggled against them, and paupers and greens and prophets bound them in lifeless stones or glass or gems and sealed them in with names of power, for they could not be destroyed, merely bound and sealed up." "The Blackstar," Henry said. The king nodded.—p. 381 The Chestnut King hardcover edition.