The London Sanctum was one of the sanctuaries of the Masters of the Mystic Arts alongside the New York Sanctum and the Hong Kong Sanctum. It was used by them to store and secure mystical items and generate a shield around the world, in order to protect it from mystical dangers.
Kaecilius and the Zealots went to the Kamar-Taj library with the intent of stealing a spell to bring Dormammu to Earth. The Zealots strung up the librarian and Kaecilius beheaded him with his Scythe Daggers before ripping a spell from the Book of Cagliostro which would allow them to complete their plans.
Before they could leave, the Ancient One herself appeared, warning that whatever plan Kaecilius had would bring him and his followers nothing but pain and sorrow. Instead of fighting, Kaecilius fled with his followers and teleported them to London throughthe London Sanctum where they ran down the streets hoping to get away from the Sorcerer Supreme.
Kaecilius and the Zealots attacked the London Sanctum, battling and eventually killing it's protector, Sol Rama before they destroyed the Sanctum which ripped a massive explosion through Kamar-Taj and destroyed the room and pillar the Eye of Agamotto was stored. Given that the room was later restored, meaning Strange reversed the damage with the Eye, hints he may have used the Eye to restore the London Sanctum as well.
- "Sanctum sanctorum" is a Latin translation of the biblical term "Holy of Holies".