Ubayd Allah al-Mahdi Billah was the founder of the Fatimid Caliphate, which emerged in the 10th century. He proclaimed himself the Mahdi, a messianic figure in Islamic belief, and established a new dynasty that claimed descent from Fatimah, the daughter of the Prophet Muhammad. His reign began in 909 CE in North Africa. Under his leadership, the Fatimid Caliphate expanded significantly, eventually reaching regions such as Egypt and Syria. Al-Mahdi Billah's rule was marked by the promotion of trade, culture, and the establishment of Cairo as a major center of learning and commerce. His legacy influenced the political and religious landscape of the Islamic world.