Hasan-i Sabbah takes Alamut

Hasan was born at Qom in the 1050s into a Twelver Shi'ite family and grew up at Rayy, where at seventeen he converted to Ismailism and swore allegiance to the Fatimid caliph al-Mustansir. The head of the Ismaili mission in western Iran, Abd al-Malik ibn Attash, sent him to Cairo, where he arrived in 1078; there he fell out with the vizier Badr al-Jamali and was banished. Back in Iran he preached in the Shi'ite highlands of Daylam, south of the Caspian, and chose as his base the castle of Alamut, set on a rock above a fertile valley of the Alborz mountains. Nizam al-Mulk had ordered his arrest, but his missionaries quietly won over the villagers and part of the garrison. Hasan himself slipped into the castle on the eve of Wednesday, 4 September 1090, according to the 13th-century historian Juvayni, and lived there for a while disguised as a teacher; when his identity came out, the garrison obeyed him. The castle's Zaydi lord, Mahdi, was allowed to leave and was given a draft for 3,000 gold dinars as the price of the fortress.

In less than two years, in 1091, Ismailis seized several towns in Quhistan in eastern Iran, and Hasan had in effect founded an independent state in the midst of the Seljuk sultanate. In 1092 Seljuk armies were sent against Rudbar and Quhistan, but the campaigns broke off after Nizam al-Mulk was stabbed to death in October 1092 — by the usual account, by an Ismaili fida'i, one who offers his life — and Sultan Malik-Shah died a few weeks later. When al-Mustansir died in 1094 and the vizier al-Afdal put his younger son al-Musta'li on the throne, Hasan recognised the elder, Nizar, as imam and broke with Cairo; the Ismailis of Iran and Iraq followed him and became known as Nizaris.

Hasan made Alamut impregnable, improved the irrigation of its valley so that it could feed itself and built up a library; he directed the movement from the castle, leading an austere life, until his death in 1124. The Nizari state, a network of fortresses in Iran and later in Syria, lasted until the Mongols took Alamut in 1256. Its enemies called the Nizaris hashishiyya, the source of the European word "assassin", and hostile writers and Marco Polo spread tales of drugged killers trained in a secret garden of paradise, which modern scholars treat as legend; so too the story that Hasan, Nizam al-Mulk and Omar Khayyam had been schoolfellows.