Disillusion with the Ancient One
Having lost his wife due to a tragic, neurologic disease, Kaecilius joined the Masters of the Mystic Arts. The leader of the order, known as the Ancient One, promised him to expand his spiritual knowledge, so that he could finally get over the pain resulting from the loss of his family. However, despite becoming quite a talented mystic artist, Kaecilius was not satisfied with the knowledge his mentor shared with him. After confronting her, but once again being denied, he decided to look for answers by himself.
He approached some of his fellow trainers, who also believed they had not found out the "truths" they were promised by the Ancient One, and offered them to teach the answers they were looking for. Together, they formed the Zealots and declared war on the Ancient One.
Infiltrating the Kamar-Taj library, Kaecilius and the Zealots subdued and beheaded the librarian before they proceed in stealing the pages containing a forbidden ritual of the Dark Dimension from the Book of Cagliostro and fled the compound when the Ancient One showed up.
Cornered by the Ancient One's Mirror Dimension in London, Kaecilius and his followers fought their former teacher, but several lost their lives in the one-sided skirmish and forced them to escape, leaving only Kaecilius, Lucian and three other Zealots left.
Empowering the Zealots
When Kaecilius happened to decipher the spell from the Book of Cagliostro, the spell empowered Kaecilius and the Zealots. The spell also gives new powers and abilities to Kaecilius and the Zealots to defeat the Master of the Mystic Arts and destroy the Sanctum Sanctorums.
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Gaining Immortality in the Dark Dimension
Unknown to the surviving Zealots, Doctor Strange had not surrendered to Dormammu, but rather had bargained with the being to release themselves from a time loop in exchange for leaving the Earth plane alone. As part of the deal, Kaecilius and his surviving disciples were given the immortality they had wanted all along. Unfortunately for them, the warnings they failed to heed turned out to be correct; the three were transformed into Mindless Ones and dragged into the Dark Dimension, condemned to an eternity of torment.