The Assassin's Order, also known as the Nizari Isma'ili, was founded by Hassan-i Sabbah around 1090 after securing the stronghold of Alamut. And this is the basis for the Assassin's Brotherhood of Assassin's Creed.
Assassin's Creed also has you square off against the Knights Templar, which this video delves into the real conflict (or lack-thereof) between the Assassin's and Knights, as well as comparing the actual assassination targets from the game to some of their real life counterparts.