Baron Karl Amadeus Mordo was a fictional villain created by Stan Lee and Steve Ditko, in comic books featuring his opponent Doctor Strange. He first appeared in Strange Tales.
Originally a Transylvanian nobleman (born in Varf Mandra), Mordo became a student of the Tibetan sorcerer known as the Ancient One. When Mordo plotted to kill his teacher, the Ancient One's newer student Dr. Stephen Strange learned of the plot, and was forced to develop great magical skills in order to overcome Mordo. Mordo was exiled by the Ancient One. Mordo's abilities were similar to those of Dr. Strange, but Mordo was particularly skilled at astral projection and mesmerism. He was more than willing to use powerful black magic and invoke demons, both of which Strange was reluctant or unable to do, although both acts would sometimes backfire on Mordo.
The evil Mordo became an open foe of Doctor Strange, serving as one of Strange's primary antagonists early in his career as Sorcerer Supreme. Mordo disguised himself as Sir Clive Bentley, and trapped Doctor Strange. He opposed Dr. Strange's discipleship to the Ancient One. He set a series of new traps for Strange in a bid to wrest magical secrets from the Ancient One. Mordo stole Strange's body while Strange was astral projecting. He imprisoned the Ancient one. Mordo made a deal with his new master, the demonic Dormammu, to amass additional power to defeat Doctor Strange. He dispatched agents to search for the incognito Strange, and then banished him from the Earth. He fled from Strange, and then dispatched agents to kill Strange. He engaged in personal combat with Strange. Mordo was exiled to the Dimension of Demons by Dormammu. He battled Dr. Strange once more at Stonehenge, but was ultimately banished from Earth.
Mordo would return in time to continue to bedevil Doctor Strange. He impersonated Doctor Strange during Strange's brief retirement, but was soon vanquished. Mordo discovered the Book of Cagliostro, and battled Strange in 18th-Century Paris. He then accompanied Strange and Sise-Neg to the dawn of time. After suffering a mental breakdown, he was placed in Doctor Strange's care, but later escaped. Mordo transformed the dead Lord Phyffe into Azrael, angel of death, and dispatched him against Strange. He sent the Man-Thing to kill Strange, and assembled thirteen people for human sacrifice to the Chaos Demon. Mordo was defeated by Strange, Jennifer Kale, and the Man-Thing. He attacked Strange, and escaped into the 1940s, but was manipulated by Dormammu.
Mordo later sold his soul to both Mephisto and Satannish for power, gaming that Strange would save him. He was trapped with Sara Wolfe, and later rescued her. Mordo himself was later imprisoned, and Sara Wolfe freed him from imprisonment. Mordo battled Dormammu to defend the Earth. He was defeated, and pretended to ally himself with Dormammu, and then allied himself with Umar to defeat Dormammu.
Umar and Mordo were eventually deposed by Clea Mordo eventually contracted terminal cancer as a side effect of his use of black magic, and renounced evil just before his death. He later returned to life.