A demon's slave is a mageborn who has given himself to a demon or a spirit, usually in exchange for some type of boon. In Sisren's Betrayal, the demon's slave is finally revealed as Councilmember Delthen's former apprentice, Ungo. Ungo dedicates himself to Vosper, and as a result, his powers grow, but he is bound to obey Vosper's whims and commands.
Every demon's slave is identified with some kind of mark. The rune-scar in Ungo's left palm is identified as the source of much of his power.