Ali al-Ridha

Ali ibn Musa al-Rida succeeded his father as Imam in 799, known among followers for scholarship and piety - works on theology and jurisprudence bear his name, and a comprehensive collection preserves his debates and sayings. He lived under three Abbasid caliphs, the dynasty that had long opposed Shia movements. Then in a startling reversal, al-Ma'mun summoned him to Merv and named him successor to the caliphate, possibly hoping to quiet the revolts; al-Rida accepted on condition he stay clear of governance. Baghdad erupted, the Abbasids installed a rival caliph, and al-Ma'mun turned his retinue toward the capital with al-Rida in tow. The Imam died in September 818 when they reached Tus, shortly after the caliph's Persian vizier - architect of the pro-Shia tilt - was assassinated. A city grew around his grave, Mashhad, now the holiest site in Iran and visited by millions each year.