The Assassins were a heretical Muslim sect. We think of them mostly in connection with political murder (their founder, Hasan-i-Sabbah, has been compared to Osama bin Laden), but there is much more to them than this. They had a remarkable esoteric philosophical system and their ideas were influential in Islam and even outside it. In this book I tell their story, from their foundation at the end of the eleventh century to their downfall 150 years later at the hands of the Mongols. Even that was not the end of them, for the Aga Khan is a lineal descendant of the Assassin Grand Masters.