Baron Karl Amadeus Mordo was the son of the mad sorcerer Baron Nikolai Mordo and the sorceress Baroness Sara who had only married Nikolai in hopes of stealing his magical secrets. Nikolai eventually found himself killed by Sara's father Krowler which left Karl to swear revenge.
Baron Mordo eventually became a pupil of the Ancient One but Mordo only used him power and attempted to betray him only to be foiled by Doctor Strange who would go on to become Mordo's nemesis. Mordo became the disciple of the evil cosmic being Dormammu who Mordo would regularly attempt to summon in hopes of Dormammu conquering the cosmos and making Mordo his lieutenant but Mordo failed time and time again at the hands of Doctor Strange.
Karl Mordo was member of the Masters of the Mystic Arts and a close ally to the Ancient One, assisting her with recruiting and training their new soldiers. When Mordo encountered Stephen Strange, he invited him into Kamar-Taj despite the Ancient One's objections and later aided him in battle against Kaecilius and the Zealots. During this battle, Mordo learned the Ancient One drew her power from the Dark Dimension, which had been forbidden from all other Masters. Following the death of the Ancient One, and the defeat of Kaecilius, Mordo was so mortified by this betrayal that he turned his back on his allies and chose to use his skills to restore balance by stripping sorcerers of their power.