Assassins
The Assassins, also known as the Hashashin, are an order of assassins, who are in a constant battle with the Knights Templar. The order has existed in many eras, including the Third Crusade, the Renaissance, and even lasting into the year 2012. The order believes in a creed of values, consisting of three tenets: Do not harm the innocent, be discreet by hiding in plain sight, and do not compromise the brotherhood.
Known Assassins
Assassin's Creed
In 1191, during the Third Crusade, the Assassin order has its headquarters in the fortress of Masyaf. Led by Al Mualim, Altair and the other assassins battle with the Templars and their leader, Robert de Sable.
Assassin's Creed 2
It has been announced in the May 2009 issue of GameInformer that the Assassins have "gone underground", but the order still continues their battle with the Templars. Ezio, who was announced as the game's protagonist, is an Italian nobleman who joins the order for an unknown reason.
Real History
The Hashashin were a real organization of assassins around the time of the Middle Ages, and did have a base in Masyaf.