in Middle Eastern and Asian history, any member of the Nizārī Ismāʿīlites, a religiopolitical Islāmic sect dating from the 11th to the 13th century and known, in its early years, for murdering its enemies as a religious duty. The Arabic name means “hashish smoker,” referring to the Assassins' alleged practice of taking hashish to induce ecstatic visions of paradise before setting out to face martyrdom. The historical existence of this practice, however, is doubtful. The stories that were told by Marco Polo and other travelers about the gardens of paradise into which the drugged devotees were introduced to receive a foretaste of eternal bliss are not confirmed by any known Ismāʿīlite source.

The Assassins were a product of dynastic strife among the Fāṭimids (Fāṭimid Dynasty), who were the heads of the Shīʿite Ismāʿīlite movement and had set up a rival caliphate in Egypt in opposition to that of the ʿAbbāsids in Baghdad. After the death of the Fāṭimid caliph al-Mustanṣir (1094), Ḥasan-e Ṣabbāḥ and other Ismāʿīlites in Iran refused to recognize the new Fāṭimid caliph in Cairo and transferred their allegiance to his deposed elder brother, Nizār, and the latter's descendants. There thus grew up the sect of the Nizārī Ismāʿīlites, who were at odds with the Fāṭimid caliphs in Cairo and were also deeply hostile to the ʿAbbāsids. The Nizārīs made many changes in Ismāʿīlite doctrine, the most significant, from the point of view of the outside world, being the adoption of terrorism as a sacred religious duty.

The open history of the Assassins began in 1090, when Ḥasan and his allies captured the hill fortress of Alamūt near Kazvin, Iran. From this centre, by the end of the 11th century, Ḥasan, as grand master or leader of the sect, commanded a chain of strongholds all over Iran and Iraq, a network of propagandists, a corps of devoted terrorists, and an unknown number of agents in enemy camps and cities. The Seljuq sultanate's attempts to capture Alamūt failed, and soon the Assassins were claiming many victims among the generals and statesmen of the ʿAbbāsid caliphate, including two caliphs.

In the early 12th century the Assassins extended their activities to Syria, where the expansion of Seljuq rule had created a favourable climate for terrorist activities by extremist elements among the local Shīʿite minority. After a period of preparation, the Assassins seized a group of castles in the An-Nuṣayrīyah Mountains, the most important of which was Maṣyāf. From this fortress the Syrian grand master, the legendary Rashīd ad-Dīn as-Sinān, ruled virtually independently of Assassin headquarters at Alamūt. Rashīd and his successor chiefs were known as the shaykh al-jabal (Arabic: “mountain chief”), which was mistranslated by the crusaders as the “Old Man of the Mountain.”

Assassin power came to an end as the Mongols under Hülegü captured Assassin castles in Iran one by one until in 1256 Alamūt itself fell. The Syrian castles were gradually subjugated by the Mamlūk sultan Baybars I and placed under Mamlūk governors. Henceforth the sect stagnated as a minor heresy. Its followers are still to be found in Syria, Iran, and Central and South Asia, with the largest group in India and Pakistan, where they are known as Khōjās (Khōjā) and owe allegiance to the Aga Khan. The term “assassin” was brought by the crusaders from Syria to Europe, where it acquired its present meaning of one who murders a politically important person either for hire or from fanatical motives.