Assassin's Creed first introduced Altaïr Ibn-La'Ahad as its main historical protagonist.
Altaïr is a Syrian Assassin operating during the Third Crusade in the late twelfth century. After failing a mission, he is demoted by his mentor Al Mualim and must complete a series of investigations and assassinations to regain his rank.
The original game was released in 2007 and presents historical locations including Masyaf, Jerusalem, Acre, and Damascus. Altaïr's story also establishes important series concepts, including the Assassin insignia, social stealth, Eagle Vision, and the ideological conflict with the Templars.
Altaïr later appears in Assassin's Creed Revelations, but that later game revisits and concludes parts of his life. It did not introduce him to the series.