Al-Musta'li in the context of Mustaali

Musta'li Isma'ilism (Arabic: المستعلية, romanized: al-Mustaʿliyya) 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 (r. 1036–1094/1095). 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.

The Musta'li originated in Fatimid-ruled Egypt, later moved its religious center to Yemen, and gained a foothold in 11th-century Western India through missionaries.

Abu Ali al-Mansur ibn al-Musta'li (Arabic: أبو علي المنصور بن المستعلي, romanized: Abū ʿAlī al-Manṣūr ibn al-Mustaʿlī; 31 December 1096 – 7 October 1130), better known by his regnal name al-Amir bi-Ahkam Allah (Arabic: الآمر بأحكام الله, romanized: al-Āmir bi-Aḥkām Allāh, lit. 'The Ruler Who Executes God's Decrees') was the tenth Fatimid caliph, ruling from 1101 to his death in 1130, and the 20th imam of the Musta'li Isma'ili branch of Shia Islam.

Al-Amir succeeded his father, al-Musta'li, at the age of five. For the first twenty years of his reign, al-Amir was a puppet of his uncle and father-in-law, the vizier al-Afdal Shahanshah, who ruled the Fatimid state and confined al-Amir, like al-Musta'li before him, to the palace. In December 1121, al-Afdal was murdered, officially by agents of the rival Nizari branch of Isma'ilism, although some medieval accounts blame al-Amir and al-Afdal's chief secretary, al-Ma'mun al-Bata'ihi, instead. Al-Amir and al-Bata'ihi moved quickly to forestall a succession by one of al-Afdal's sons, imprisoning them and moving the vast treasures al-Afdal had amassed into the caliphal palace. Al-Bata'ihi was appointed as the new vizier, but al-Amir took an increasing role in government, and was prominently featured in spectacular public ceremonies. Finally, in 1125, al-Amir dismissed and imprisoned al-Bata'ihi, ruling thenceforth without a vizier.