The legendary sword wielded by Altaïr in Assassin's Creed Revelations is called Altaïr's Sword.
In Revelations, Altaïr's Sword is a unique weapon associated with Altaïr Ibn-La'Ahad, the legendary Assassin whose memories are explored through the game's story. The weapon represents Altaïr's status and legacy within the Brotherhood, while the modern storyline follows Desmond and the main historical campaign follows Ezio.
Assassin's Creed Revelations was released in 2011 and is set mainly in Constantinople during the early sixteenth century. Ezio travels there to seek the Masyaf Keys, which unlock Altaïr's library. The game uses these keys to connect Ezio's later life with Altaïr's earlier career.
The answer is easy to confuse with the Sword of Eden, a Piece of Eden associated with other characters and stories. The Apple of Eden is another type of artifact, and the Masyaf Keys are objects used to open the library rather than swords.