Comic book supervillain, created by Stan Lee and Steve Ditko for Marvel Comics. He made his first appearance in Strange Tales #111 in August 1963 and is generally considered one of Doctor Strange's arch-enemies.
His full name is Karl Amadeus Mordo. A Transylvanian nobleman -- but not a vampire! -- he sought to learn to become a sorcerer and magician, traveling to Tibet to study the mystic arts under Earth's Sorcerer Supreme, the Ancient One. Mordo plotted to kill his master, but the scheme was discovered by Doctor Strange, who attempted to warn the Ancient One. Mordo cast spells restraining Strange, but luckily, the Ancient One was in no danger. You don't live several centuries and become Earth's foremost wizard without being a quick thinker -- the Ancient One had been aware Mordo's treachery all along.
Though Mordo was cast out of Tibet, he continued to be an ongoing foe of Strange and the Ancient One. His skills were similar to Strange's, but generally a bit weaker. He was, however, skilled in the use of black magic and the summoning of demons, which Strange was reluctant to do. He was also a regular ally of the monstrously powerful Dormammu.
Mordo clashed with Strange frequently, suffering a variety of punishments and indignities. He was banished from Earth, trapped in a dimension full of demons, sold his soul to both Mephisto and Satannish, had a nervous breakdown, got lost in time, and died and returned to life several times. He defeated Strange a few times, but never for very long. He's basically Doctor Strange's oldest foe, but he's usually been more of a punching bag than a real threat.
Mordo appeared in the 2016 "Doctor Strange" film, in which he was played by British actor Chiwetel Ejiofor. In the film, Mordo was an ally of Strange and the Ancient One and even assisted with Strange's training as a sorcerer, as well as aiding in the battle against the film's Big Bad, Kaecilius. He becomes disillusioned by the Ancient One's willingness to compromise her principles and eventually decides Earth has too many sorcerers, setting him up as a future antagonist in the film series.