al-Hadi ila'l-Haqq Yahya

Yahya ibn al-Husayn, known by the regnal title al-Hadi ila'l-Haqq ("the Guide to the Truth"), was the founder of the Zaydi imamate in the highlands of Yemen, a state whose successors endured, on and off, until 1962. Zaydism is a branch of Shia Islam that recognizes a line of imams (community leaders descended from the Prophet Muhammad) and that has historically prized rationalist theology and active resistance to unjust rulers.

A descendant of the Prophet through al-Hasan, and grandson of the noted Zaydi scholar al-Qasim al-Rassi, al-Hadi was born around 859 (245 AH). Modern scholarship places his birth at a village near the Wadi al-Rass, southwest of Medina; later Zaydi tradition says Medina itself. He won early renown as a jurist. After an unsuccessful bid for recognition in the Caspian region of Tabaristan, and a first aborted attempt in Yemen, he was invited by feuding northern tribes and reached Sa'da in 897, where he proclaimed his da'wa (his "call" to the imamate) and built a lasting base.

He campaigned to Najran and repeatedly contested Sana'a, though he never held that city securely. Beyond politics, his lasting influence is legal: his writings, especially the Kitab al-Ahkam, anchored the Hadawi madhhab (school of jurisprudence) that still shapes Zaydi law in Yemen. He died at Sa'da on 18 August 911 and is buried there. Zaydis traditionally rank him second in importance only to Zayd ibn Ali himself.