Karl Mordo was the Ancient One's disciple, but he planned to destroy him and take his place as Sorcerer Supreme. His plot was thwarted with the help of Stephen Strange, who was the really worthy one to succeed the Ancient One. Since then, Strange and Mordo are bitter enemies.
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.