Xander is a supervillain from the Marvel comics. He is an enemy of Doctor Strange.
The origins of Xander are unknown. He was recruited by the Creators, who are human sorcerers from a variety of temporal periods on Earth. However, they have access to a number of different dimensions, and it appears that Xander does not originate from Earth. He is referred to on one occasion as an "alien". The Creators trained Xander to be their herald, teaching him mastery of various forms of magic, as well as a number of forms of combat.
The Creators summoned Xander to send him against Dr. Strange, whom they saw as a threat to their goals of Universal domination. Xander (and the Creators) manipulated the Ancient One, causing him to strip the title and power of Sorcerer Supreme from Dr. Strange.
Xander attacked Dr. Strange and Clea. He instantly rendered Clea unconscious, while simultaneously placing a spell on her to manipulate her future actions. Xander battered Strange with a relentless onslaught of magical attacks, which Strange just barely managed to survive. Xander realized that he would need to call on his full power and prepared to summon power directly from the stars themselves. As he was focusing his powers, Strange ran up and punched him out. The Creators recalled their defeated warrior.
Clea lost her memory as a result of Xander's assault and ended up going on a rampage through Manhattan. Strange was called away on another mission, but returned home and dispersed Xander's spell, restoring her to normal.