Abū Muḥammad ʿAbd Allāh ibn al-Ḥusayn, better known by his regnal name al-Mahdī biʾllāh, was the founder of the Isma'ili Fatimid Caliphate, the only major Shi'a caliphate in Islamic history, and the eleventh Imam of the Isma'ili branch of Shi'ism. Wikipedia