In Assassin’s Creed, the Apple of Eden is the Apple-shaped Piece of Eden sought by Altaïr.
Altaïr Ibn-La’Ahad encounters the artifact during the Third Crusade after the Assassins and Templars compete for control of ancient technology created by the Isu. The Apple appears to provide powerful effects, including illusions, influence over minds, and access to visions connected with the past.
The artifact is central to the first game’s conflict because both sides believe it can change the balance of power. Altaïr’s investigation eventually reveals that Al Mualim, the Assassin mentor in Masyaf, has kept the Apple and intends to use it himself.
The Apple is one of several Pieces of Eden in the wider series. The Sword, Staff, and Shroud are different artifacts with separate histories and abilities. The word “Apple” identifies this particular spherical artifact, not every Piece of Eden or every Isu relic.