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The Nizari movement emerged during the reign of Al-Musta'li, the sixth Fatimid caliph. This split occurred due to a succession dispute following Al-Musta'li's death, with the Nizari faction supporting his son Nizar as the rightful heir. The Nizari movement became a significant branch of Shia Islam, known for its distinctive beliefs and practices, including the recognition of Nizar and his descendants as the rightful imams.