Musta'li Isma'ilism is a branch of Isma'ilism named for their acceptance of al-Musta'li as the legitimate ninth Fatimid caliph and legitimate successor to his father, al-Mustansir Billah. The Nizari the other living branch of Ismailism, led by Aga Khan V believe the ninth caliph was al-Musta'li's elder brother, Nizar.