In 2016, Kaecilius, a trusted member of the Masters of the Mystic Arts who was determined to understand why his wife and unborn son died, lost faith in the Ancient One after he discovered that his mentor was drawing power from the Dark Dimension.
Viewing this as an act of betrayal, Kaecilius convinced some of his fellow Masters to aid him in his journey to seek the truths that their mentor had long denied them. He led his newfound followers in an assault on Kamar-Taj, during which they stole an ancient ritual that would allow the caster to summon the entity Dormammu.
Months later, Kaecilius successfully deciphered and performed the ritual, granting both him and the Zealots complete control over the Dark Dimension and Mirror Dimension. The Zealots then set out to orchestrate a full-scale attack on their former allies, beginning with an assault on the London Sanctum.
After executing Sol Rama, the Sanctum's protector, the separatist faction caused an explosion that not only destroyed the Sanctum, but ripped through Kamar-Taj. In the ensuing chaos, new member Stephen Strange was sent through the portal leading to the New York Sanctum.
Making his way through the Sanctum, Strange came across Kaecilius and two of his fellow Zealots fighting Daniel Drumm in the main foyer. Just as Kaecilius was preparing to deliver the fatal blow, Strange made his presence known from the second-floor landing.
Not recognizing the Ancient One's latest recruit, Kaecilius asked Strange how long he had been at Kamar-Taj only to start an awkward conversation about the latter's name. The lead Zealot proceeded to execute Drumm and bring the conversation to an abrupt end before charging towards his newest enemy.
Strange attempted to evade the Zealots by running down the hallway leading to the Rotunda of Gateways only for Kaecilius to manipulate the corridor and place it in a kaleidoscopic state.
Despite struggling with his Eldritch Whip and Tao Mandalas, Strange successfully overpowered Kaecilius' right-hand man Lucian and the other remaining Zealot. Strange then sent the Zealots through the portals respectively leading to the rainforest and the desert.
Strange proceeded to fight Kaecilius and threw him into a glass case holding the Cloak of Levitation before reaching for an axe. The Cloak, however, noticed the Crimson Bands of Cyttorak on the other side of the room and convinced Strange to use it instead.
While unable to understand the device, Strange threw it at Kaecilius before watching as the bands trapped and subsequently restrained the lead Zealot.
Kaecilius convinced Strange to remove the device's mouthpiece and revealed his master plan to "save" Earth by merging it with the Dark Dimension, thereby granting the people of Earth eternal life.
After revealing the Ancient One's secret, the lead Zealot burst into laughter. When Strange inquired as to what he was laughing about, Kaecilius noted it was because Strange did not have his Sling Ring.
At that exact moment, Lucian, who managed to escape from the rainforest, stabbed Strange in the back with a Space Shard and began to fight him. The Cloak of Levitation came to Strange's aid and wrapped itself around Lucian's face, allowing Strange to find his Sling Ring and open a portal to Metro-General Hospital.