The Theft of the Book of Cagliostro was a successful act of infiltration undertaken by former Masters of the Mystic Arts member Kaecilius and his followers the Zealots into the Kamar-Taj library to obtain the ritual from the Book of Cagliostro which would not just grant them power from the Dark Dimension but also allow its ruler Dormammu access to the Earth and its dimension. After locating and stealing the necessary pages pertaining to the ritual, Kaecilius and the Zealots were confronted by the Ancient One who engaged them in battle in the Mirror Dimension.
Having lost his wife and unborn son, Kaecilius desperately sought to understand why his family died. This led him to Karl Mordo, who introduced him to a Celtic sorcerer known only as the Ancient One. Under her tutelage, Kaecilius became a member of the Masters of the Mystic Arts.
Eventually, Kaecilius began to feel unsatisfied with the knowledge his mentor had shared with him. After learning that several other Masters were also unsatisfied, Kaecilius formed the Zealots with the sole purpose of seeking the truths that the Ancient One had denied them.
Remembering a ritual that would allow the Zealots to summon the entity Dormammu, Kaecilius planned to steal it from the Book of Cagliostro, which was kept under close observation in the library of Kamar-Taj.
BattleZealots overpowered and restrained the librarian before watching as Kaecilius calmly decapitated him with his Scythe Daggers. He then located the Book of Cagliostro and found the ritual, which he ripped out of the book.
The Ancient One suddenly appeared and advised against performing the ritual, prompting Kaecilius to open a portal so he and the Zealots could escape. The Ancient One followed them through the portal to London before trapping them in the Mirror Dimension so they could not endanger innocent lives.
Sensing that the battle was lost, Kaecilius used his Sling Ring to open a portal so he could escape with his remaining followers. The Ancient One, meanwhile, restored the warped landscape before exiting the Mirror Dimension.
Several months after the theft, Kaecilius deciphered and performed the ritual, which granted the Zealots access to the Dark Dimension as well as unparalleled control over the Mirror Dimension. He then prepared to launch an attack on the three Sanctums and leave Earth vulnerable to Dormammu.