Which fictional Assassin order does Altaïr serve in the first Assassin’s Creed game?

Altaïr serves the Levantine Brotherhood in the first Assassin’s Creed game.

The Levantine Brotherhood is the Assassin organization operating under Al Mualim from the mountain stronghold of Masyaf. Altaïr begins the story as one of its senior members, but his failure during an early mission causes him to lose rank and equipment. He must then complete a series of missions before regaining his status.

The Brotherhood’s activities extend across the Holy Land, including Jerusalem, Acre, and Damascus. Its members oppose the Templars, who seek control through political and social domination. The game presents both groups as secret organizations pursuing competing visions of order and freedom.

The organization is sometimes confused with the Hidden Ones, the earlier group featured in Assassin’s Creed Origins. The Hidden Ones are the historical precursor to the later Assassin Brotherhood. In Altaïr’s era, the relevant organization is generally identified as the Levantine Brotherhood.