The original Assassins were members of a revolutionary Shiite splinter group founded in eleventh century Persia by Hassan Sabbah.
Like many schismatic religious groups, the Assassins believed that Muslims, including mainstream Shiites, had taken a wrong turn. Islam needed to go back to its foundations. As far as other Muslims were concerned, Sabbah’s beliefs were heresy. Among other things, he taught that Mohammed’s son-in-law, Ali, and the Shiite imams who succeeded him were incarnations of Allah.
Like anarchists in the early twentieth century, the Assassins had neither money nor political power so they turned to the public murder of important figures as a way of exercising influence over society. (Anarchists called this “propaganda of the deed”.) The cult was organized as a secret society. The goal was not the removal of specific political leaders, but making people believe that the Assassins could kill anyone at any time.
The sect was destroyed in 1256 when the Assassins made the mistake of trying to kill Hulagu, the grandson of the Genghis Khan and leader of the Mongol hordes. (If you want to assassinate a Chinggisid prince, you need to get it right the first time.)
Assassin n. One who undertakes to put another to death by treacherous violence. The term retains so much of its original application as to be used chiefly of the murderer of a public personage, who is generally hired or devoted to the deed, and aims purely at the death of his victim.