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, warning them that the stolen ritual would bring them sorrow.
Cornered by the Ancient One's Mirror Dimension in London, Kaecilius and his followers fought their former teacher, but most 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 gave new powers and abilities to Kaecilius and the Zealots to defeat the Masters of the Mystic Arts and destroy the Sanctum Sanctorums.
The Zealots' first attack after they were given new powers and abilities was at the London Sanctum. There, the Brunette Zealot and Kaecilius fought the Sanctum's protector Sol Rama, a Master of the Mystic Arts, in an intense duel. Rama, sensing the battle was lost, attempted to escape to Hong Kong through a portal, but he fell short and was stabbed from behind by one of the Brunette Zealot's Space Shards. Soon after Kaecilius and the Zealot destroyed the Sanctum, giving a blow to Doctor Strange in the process, who was ejected to the New York Sanctum.
Following their success in London, Kaecilius, Lucian Aster and the Blonde Zealot attacked the New York Sanctum. They murdered the guardian of the Sanctum, Daniel Drumm, before being confronted by Doctor Strange. The Zealots attacked Strange, who briefly managed to counter their assaults before Kaecilius joined the battle and used his abilities to overpower Strange. Despite that, Strange managed to strand the Blonde Zealot in a desert and Lucian in a jungle.
Remaining alone with Strange, Kaecilius kept fighting against him. Kaecilius overpowered Strange, mocking his inability to use the Brazier of Bom'Galiath and getting on the verge of killing him. However, Strange received the unexpected assistance of the Cloak of Levitation, which directed Strange to the Crimson Bands of Cyttorak, which were used to subdue Kaecilius.
While Kaecilius explained his motives all the while playing for time, Aster returned to the New York Sanctum and stabbed Strange with a Space Shard. Aster prepared to strike a final blow, but the Cloak of Levitation attacked him and Aster lost his Sling Ring to Strange, who escaped to Metro-General Hospital. Still fighting against the Cloak, Aster let his astral body to head for the hospital and deal with Strange, but he was eventually killed when Christine Palmer overloaded Strange's body with a defibrillator. In the meantime, Kaecilius successfully escaped from the New York Sanctum.
Kaecilius gathered the Brunette Zealot and the Tall Zealot and together, the Zealots resumed their attack on the New York Sanctum. Karl Mordo attempted to fight them, and as Kaecilius was on the verge of striking the final blow against the Sanctum, he and the two other Zealots were trapped in the Mirror Dimension by Doctor Strange, who stole Kaecilius' Sling Ring before running away with Mordo and attempting to leave the Zealots in the Mirror Dimension.
However, the Zealots' connection to the Dark Dimension rendered them more powerful in the Mirror Dimension. Kaecilius and the other Zealots chased Strange and Mordo to the point where Strange was almost killed by Kaecilius. Nevertheless, the Ancient One joined the battle and took both Strange and Mordo away from the fight. Kaecilius then blamed his former teacher for not having disclosed the full extent of her knowledge. After she warned him that Dormammu's promised immortality was torment, he called her a liar and attacked her with the other Zealots. As the Tall Zealot attacked the Ancient One, Kaecilius stabbed him in the back with a Space Shard, which pierced through both his body and the Ancient One's.
As the Tall Zealot fell, Kaecilius kicked the Ancient One through an Inter-Dimensional Portal, prompting Strange and Mordo to leave and try to rescue her, and later escaped the Mirror Dimension with the Brunette Zealot.
The Zealots led a final assault against the Masters of the Mystic Arts by attacking the Hong Kong Sanctum. Kaecilius, the Brunette Zealot and the Blonde Zealot defeated their opponents, killing Wong, resulting in a tear through which the Dark Dimension began to invade the Earthly Plane.
However, Doctor Strange and Karl Mordo intervened in the fight. By using the powers of the Eye of Agamotto, Strange reverted time, which repaired all the damage caused by the fight between the Zealots and the Masters of the Mystic Arts. However, the Zealots broke free of the time reverting process, enabling them to fight against Strange and Mordo. Although Kaecilius and the two other Zealots were momentarily trapped in buildings being restored, Kaecilius managed to break free and interrupted the spell, which rendered time still.
Joined by the two other Zealots, Kaecilius prepared to resume the fight against Strange, Mordo and a revived wrong, but before they could proceed, Strange left to enter the Dark Dimension. Kaecilius mocked Mordo and Wong, claiming that even Strange had succumbed to Dormammu's promise of eternal life.
Unbeknownst 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 Earthly 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 as the three were transformed into Mindless Ones and dragged into the Dark Dimension, condemned to an eternity of torment.