Al Mualim is The Mentor of the Levantine Assassin Order. He gives Altaïr the mission to assassinate nine corrupt head figures of the Holy Land as a way for him to earn his redemption.
In reality, he was a high-ranking member of the Templars, if not their leader. Once Al Mualim had the Piece of Eden, he sent Altaïr to deal with the other nine Templars in the Holy Land to ensure he would remain unchallenged. Fortunately, Altaïr killed him once he discovered this treachery.