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Kaecilius, played by Mads Mikkelsen, is the main antagonist in the 2016 Marvel film "Doctor Strange." Kaecilius is a former disciple of the Ancient One, who turns against her and seeks to use dark magic to access other dimensions and gain immortality. He leads a group of renegade sorcerers known as the Zealots, who believe the use of dark magic is necessary to protect the Earth from mystical threats.