The Destruction of the London Sanctum was the first stage of an attack on the Sanctums orchestrated by the Zealots in their quest to summon Dormammu.
Thousands of years ago, a sorcerer known as Agamotto formed the Masters of the Mystic Arts as a league of magic practitioners dedicated to protecting Earth from any inter-dimensional threats.
To aid his disciples in their mission, the Sorcerer Supreme travelled to three different places of power and erected three sanctums that would generate a protective shield around the Earth. The sorcerer then charged his disciples with guarding the three sanctums.
By the year 2016, the three places of power had evolved into the cities of Hong Kong, New York City, and London. The Masters of the Mystic Arts had also evolved, its ranks expanding to include members such as Karl Mordo.
It was around the same time that Kaecilius, a member who joined in order to understand why his wife and unborn son lost their lives, became disillusioned and persuaded several other Masters to join him in his quest to seek the answers the Ancient One, the current Sorcerer Supreme, had denied them.
Remembering a ritual that would allow him to summon the entity Dormammu, Kaecilius and the newly-formed Zealots stole it from the Book of Cagliostro despite the Ancient One's warning.
Several months later, Kaecilius managed to decipher and perform the ritual, which granted both him and the Zealots complete access to the Dark Dimension, before preparing to launch a series of attacks on his former allies.