Hassan Al-Sabah, Death Toll: maybe thousands. The Assassins were one of history's famous death cults. It was founded by Hassan Al-Sabah, the "old man of the mountain", who ran his sect from the mountain fortress of Alamut. His cult was properly called the "Hashshashin", and they belonged to a particular sect of Islam known as the Ismailis, a subset of Shia Islam. This is a particularly esoteric and mystical sect of Islam, and today they're known as being relatively moderate, and their modern leader (the Aga Khan) is known for his charity work and promotion of secular pluralism. But in the 11th century, the Ismailis were religious warriors engaged in a struggle with their Sunni Muslim rivals (as well as with Christian crusaders) for control of the Middle East. The Assassins were dedicated to targeting and killing enemies of Ismailism, particularly prominent Sunni commanders, scholars, and political leaders. Legend has it that Assassins were brought into the order at a young age, trained in religious doctrine, subterfuge, combat and assassination techniques, and received special initiations that may have involved the consumption of hashish to receive visions of paradise. From his base on the mountain, the Old Man would send his assassins to terrorize and murder anyone who threatened the Ismaili cause. In fact, the Assassins outlived Hassan, and there was more than one "old man of the mountain", as the successive leaders of the order took on that title, giving the leadership of the assassins the appearance of immortality. The Assassins met their doom when they tried to murder an invading Mongol Khan; the assassination attempt failed, and the Mongols besieged the Assassin's mountain fortress, killing all those inside.