By night Hassan-i Sabbah like a civilized wolf in a turban stretches out on a parapet above the garden & glares at the sky, conning the asterisms of heresy in the mindless cool desert air. True, in this myth some aspirant disciples may be ordered to fling themselves off the ramparts into the black--but also true that some of them will learn to fly like sorcerers.
-Hakim Bey, The Temporary Autonomous Zone

Founder of the Isma'ili Shi'i sect known to posterity as the Assasins or hashishim. His supporters claimed that he was a descendant of the Fatimid caliph in Cairo, and thus of the direct line of Ali, making his a legitimate shi'i imam, qualified to speak inerrantly on matters of Islam and Islamic law. This claim is dubious at best, but important for understanding the history of the assasins.

Hassan-i Sabbah reorganized the assasins, moving them to the Persian mountain fortress of Alamut (in English, The Eagle's Nest), where they were out of reach of most of the sectarian violence which was slowly dismembering the old Islamic Empire. He made liberal use of the Shi'i tradition called taqiyya, or permissible dissmulation, which meant that a believer could lie about their beliefs to preserve their lives or further the cause of the faith, an idea antithetical to Christian and Sunni thought of the time. This was the kernel of the famous assasin custom which gained their name its modern notoriety.

His (possibly step-)son, Hassan-i Sabbah II made a pronouncement that for his people "the chains of law have been broken", abandoning the sharia, or traditional Islamic law, entirely. This is symbolized in traditional literature by him "breaking his fast with wine", which is to say, drinking wine, forbidden to Muslims, during the Ramadan fast.

The assasins still exist to this day, as the Ishmaelians. They now live in the hills of Northern India, though in stark contrast to their myth, they practice total pacifism.

Update: I read in the paper today that the Agha Kahn, the spiritual leader of the Ishmaelian shi'i movement and direct-line successor to Hassan-I Sabbah, was the pallbearer at Pierre Trudeau's funderal. I have no idea what this means, but, as a wise man once said, "The theological implications are startling."