Hasan-i Sabbah (c. 1050 – 12 June 1124), also known as Hasan I of Alamut, was a military leader, founder of the Nizari Ismai'li sect, Order of Assassins, as well as the Nizari Ismaili state, ruling from 1090 to 1124 AD.
Alongside his role as a leader, Sabbah was a scholar of mathematics, most notably in geometry, as well as astronomy and philosophy, especially in epistemology. It is narrated that Hasan and the Persian polymath Omar Khayyam were close friends since their student years. He and each of the later Order of Assassins' leaders came to be known in the West as the "Old Man of the Mountain", a name given by Marco Polo that referenced the sect's possession of the commanding mountain fortress of Alamut Castle.
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