leader of an Islamic sect, the Nizārī Ismāʿīlites, and commonly believed to be the founder of the order known as the Assassins (Assassin).

Ḥasan studied theology in the Iranian city of Rayy and at about the age of 17 adopted the Ismāʿīlite faith. He was an active believer and rose in the Ismāʿīlite organization. In 1076 he went to Egypt, probably for further religious training, remaining there for about three years. When he returned to Iran he traveled widely in an effort to further Ismāʿīlite interests. He made a number of converts, and, in 1090, with the aid of converts made within its garrison, was able to seize the great fortress of Alamūt in Daylam, a province of the Seljuq empire. After further turmoil, Ḥasan settled down to the leadership of a territorially scattered yet cohesive state. After the last major siege of Alamūt (1118), Ḥasan was able to live out the remainder of his life in peace. He led an ascetic existence and imposed a puritanical regime at Alamūt—when one of his sons was accused of murder and the other of drunkenness, he had them both executed. He wrote a number of cogent theological treatises, stressing in particular the need to accept absolute authority in matters of religious faith. His expression of this doctrine became widely accepted by contemporary Nizārīs.