The Battle at the New York Sanctum was the first confrontation between Doctor Strange and Kaecilius after the latter destroyed the London Sanctum in an attempt to summon Dormammu.
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.