al-Musta'li bi-Allah

Abu al-Qasim Ahmad, who ruled as al-Musta'li bi-Allah ("the one raised up by God"), was the ninth Fatimid caliph of Egypt and, in the reckoning of the Musta'li branch of Ismaili Shi'ism, its nineteenth imam. The Fatimids were a Shi'i dynasty that ruled from Cairo, claiming descent from the Prophet Muhammad through his daughter Fatima. Al-Musta'li was most likely the youngest son of the long-reigning caliph al-Mustansir, and was born in Cairo, traditionally dated to 1074 (the year is given variously and is not certain).

When al-Mustansir died in 1094, the powerful military vizier al-Afdal — al-Musta'li's brother-in-law — placed him on the throne over his elder brother Nizar. Ismaili tradition holds that succession rests on nass, a binding designation by the previous imam; Nizari Ismailis maintain that al-Mustansir had designated Nizar and that such a designation cannot be revoked, while Musta'li tradition holds that a later designation fell to Ahmad. The truth of this contested matter is held differently by the two communities. Nizar rebelled in Alexandria, was defeated, and died in captivity — a rupture that permanently split the movement into Musta'li and Nizari lines that persist to this day.

Throughout his reign al-Musta'li remained subordinate to al-Afdal. During these years the First Crusade reached Syria and captured Jerusalem (1099), and Fatimid forces were defeated at Ascalon. He died in Cairo in 1101, by some reports poisoned, and was succeeded by his young son al-Amir. The later Tayyibi and Bohra communities trace their imamate through his line.