Zealots

The Zealots are a separatist faction of the Masters of the Mystic Arts led by Kaecilius.

Disillusion with the Ancient One
Having lost his wife and unborn son 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.

Theft of the Book of Cagliostro
Infiltrating the Kamar-Taj library, Kaecilius and the Zealots subdued and beheaded the librarian before they proceed in stealing a page 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 the city, Kaecilius and his followers fought their former teacher but several lost their lives in the one-sided skirmish and forced them to escape.

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.