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Abū'l-Qāsim Aḥmad al-Musta‘lī bil-Lāh (16 September 1074 – 12 December 1101, Arabic: أبو القاسم أحمد المستعلي بالله) was the ninth Fatimid caliph, and believed by the Musta'li sect of Isma'ilism to be the nineteenth Imam.
Al-Musta‘li was made caliph by vizier al-Afdal Shahanshah and was the successor to al-Mustansir Billah. By and large, al-Musta‘li was subordinate to Malik al-Afdal. One complication with the selection of al-Musta‘li was that his older brother Nizar was considered by Nizar's supporters to be the rightful heir to the throne. This led to a power struggle within the Fatimids, and although Nizar's revolt was unsuccessful (ending with his death in prison in 1097), the break from the rules of succession caused a schism among Isma'ili Shi'i Muslims. In Seljuq-ruled Syria and Iran, the Nizari sect developed.
During al-Musta‘li's reign, the First Crusade (1099) established the Kingdom of Jerusalem, the County of Tripoli and the Principality of Antioch, which further reduced Fatimid power in Syria and Palestine. He was succeeded by his son al-Amir bi-Ahkami'l-Lah (r. 1101–1130), after whose reign the Musta'li sect again split into the Hafizi and Taiyabi; the former are extinct.
- Family tree of Muhammad#Family tree linking prophets to Imams
- List of Ismaili imams
- Lists of rulers of Egypt
- editor, Josef W. Meri, (2006). Medieval Islamic civilization : an encyclopedia. New York: Routledge. p. 405. ISBN 0415966906.
- Daftary, Farhad (2012). Historical dictionary of the Ismailis. Lanham, Md.: Scarecrow Press. p. 19. ISBN 978-0810861640.
- Daftary, Farhad (1994). The Ismā'īlīs : their history and doctrines (Repr. ed.). Cambridge [u.a.]: Cambridge Univ. Press. pp. 261–265. ISBN 9780521429740.
Al-Musta'liBorn: 16 September 1074 Died: 12 December 1101
| Fatimid Caliph
1094 - 1101
al-Amir bi'Aḥkāmi l-Lah
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