Hasan-i Sabbah, also known as Hasan I of Alamut, was a religious and military leader, founder of the Nizari Ismai'li sect widely known as the Hashshashin or the Order of Assassins, as well as the Nizari Ismaili state, ruling from 1090 to 1124 AD.

The Nizaris are the largest segment of the Ismaili Muslims, who are the second-largest branch of Shia Islam after the Twelvers. Nizari teachings emphasize independent reasoning or ijtihad; pluralism—the acceptance of racial, ethnic, cultural and inter-religious differences; and social justice. Nizaris, along with Twelvers, adhere to the Jaʽfari school of jurisprudence. The Aga Khan, currently Aga Khan IV, is the spiritual leader and Imam of the Nizaris. The global seat of the Ismaili Imamate is in Lisbon, Portugal.