Militant offshoot of the Islamic Isma'ili sect 1089-1256, founded by Hassan Sabah (c. 1045-1124). Active in Syria and Persia, they assassinated high officials in every Muslim town to further their extremist political ends. Their headquarters from 1090 was the Alamut clifftop fortress in the Elburz Mountains, NW Iran.

Their leader, Hassan Sabah, became a missionary and rebel against the Seljuk Empire after his conversion to the sect. As grand master of the Assassins, he ran the order with strict asceticism. The assassins were members of a suicide squad: they remained at the scene of the crime to be martyred for their beliefs. Their enemies called them hashishiyun 'smokers of hashish'.

Princes, viziers, and also Crusaders were among their victims.

Hassan was a scholar and Alamut, built on a peak of 1,800 m/6,000 ft, held one of the largest libraries of the time.

Hassan was born in Qom. In his twenties he may have become spymaster to the Seljuk sultan Malik Shah of Transoxania (where he met his contemporary, the poet Omar Khayyam). Banished after engaging in palace intrigue, Hassan studied in Cairo 1078-80 and then went forth as a covert missionary for the proscribed sect. Once he had obtained the castle of Alamut ('eagle's nest'), he never left it. He is said to have executed both his sons for trivial transgressions. After his death, the Assassins carried on, though they gave up austerity and assassination in 1164. In 1256 Alamut was taken by the Mongol invaders, and the library was destroyed.