Jefferson Gray’s excellent, thorough 2010 essay on the history of the Nizari Ismailis, more popularly known as the Order of the Assassins:
A small Shiite sect more properly known as the Nizari Ismailis, the Assassins were relatively few, geographically dispersed, and despised as heretics by both the Sunni Muslim majority and even by most other Shiites. By conventional standards, the Assassins should have been no match for the superior conventional military power of any of their many enemies. But near the end of the 11th century, the charismatic and ruthless Hasan-i Sabbah forged this small, persecuted sect into one of the most lethally effective terrorist groups the world has ever known. Even the most powerful and carefully guarded rulers of the age—the Abbasid and Fatimid caliphs, the sultans and viziers of the Great Seljuk and Ayyubid empires, the princes of the Crusader states, and emirs who ruled important cities like Damascus, Homs, and Mosul—lived in dread of the chameleonlike Assassin agents.
Some of the comments are, err, entertaining.