From SuspireWiki

The High Prince is the Invictus title for what most know as Overlord, but is given the function of “Seignior” in the Invictus. This title is granted by the King to an Overlord who has sworn an Oath of Fealty to him. This sometimes gives legitimacy (at least in Invictus eyes) to one Overlord over another when conflicts arise such as the case in the Viceroyalty of the Frozen North where a Sanctified Archbishop is consistently trying to usurp the High Prince’s position.