Xanthor
Xanthor's portrait from Might and Magic VIII: Day of the Destroyer.
Archmage of Erathia under Catherine.
Horn of the Abyss
He was captured and imprisoned by the barbarians and eventually rescued by Bidley. In gratitude for being saved, he told Bidley the location of Island of the Vengeful Serpent - the resting place of the Horn of the Abyss.
Lost Lore
Xanthor (along with Marcus Finch) is the narrator of a few legendary accounts of events around Antagarich during the events of Heroes of Might and Magic III. According to these stories, he is the nephew of Agar and has 'little training in the mystical arts'.