Al-Musta'li

Abū al-Qāsim Aḥmad ibn al-Mustanṣir (; 15/16 September 1074 – 11/12 December 1101), better known by his regnal name al-Mustaʿlī biʾllāh (, ), was the ninth Fatimid caliph and the 19th imam of Musta'li Ismailism.

Although not the eldest (and most likely the youngest) of the sons of Caliph al-Mustansir Billah, al-Musta'li became caliph through the machinations of his brother-in-law, the vizier al-Afdal Shahanshah. In response, his oldest brother and most likely candidate for their father's succession, Nizar, rose in revolt in Alexandria but was defeated and executed. This caused a major split in the Isma'ili movement. Many communities, especially in Persia and Iraq, split off from the officially sponsored Isma'ili hierarchy and formed their own Nizari movement, holding Nizar and his descendants as the rightful imams.