Kaecilius is the main antagonist of the 2016 Marvel Cinematic Universe film Doctor Strange. He is a former member of Masters of Mystic Arts who became a servant of Dormammu under the false promise of eternal life.
Kaecilius is basically an evil version of Doctor Strange himself as both were arrogant and joined Kamar-Taj with the hopes of regaining what they lost however Strange abandoned his path of arrogance while Kaecilius continued down the path leading him to become Dormammu's chief servant.
In the original comics Kaecilius was just a henchman of Baron Mordo.
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