A division of Shi'ism prompted by the Shi'ite/Sunni split caused after the death of the Prophet Mohammed. Twelver Shi'as believe that the Twelfth Imam was taken into Occultation, or mystical hiding, to await his return at the end of the world at the side of Mahdi and Jesus Christ, and inaugurate an era of Islamic peace.
References
Berkey, "The Formation of Islam"