The ancient artifact sought throughout the first Assassin's Creed game is the Apple of Eden.
The Apple is one of the Pieces of Eden, advanced artifacts created by the series' precursor civilization. In the first game, Altaïr Ibn-La'Ahad is ordered to recover it after it is discovered in the possession of Robert de Sablé and other Templar forces.
The artifact is portrayed as a powerful device capable of influencing minds and producing extraordinary effects. Its existence helps explain why the Assassins and Templars repeatedly compete for relics across different historical periods.
The Apple should not be confused with the Sword, Shroud, or Staff of Eden, which are separate Pieces of Eden appearing elsewhere in the franchise. The first game's central artifact is specifically the Apple, and its recovery drives the final part of Altaïr's mission.