Overlords are Hell's most powerful demons in terms of power and their ranking in Hell's hierarchy. The concept of Overlords was first introduced in the pilot during the scene where Vaggie explains Alastor's origins.
The Overlords are considered so powerful that they rule over certain parts of Hell and have an infamous reputation of doing so (in the case of both Lucifer and Lilith, they rule over ALL of Hell). They are to appear in Non Disney Heroes vs Villains in a flashback and are one of the council members for the Demon King.