The Theft of the Book of Cagliostro was an attempt made by the Zealots to infiltrate Kamar-Taj and steal a ritual that would allow them to summon Dormammu.
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.