The Apple of Eden is the ancient Isu artifact central to the plot of the first Assassin's Creed.
In the game, the Apple is a Piece of Eden, a powerful relic created by the Isu, the technologically advanced civilization that preceded humanity's known history. The artifact can influence human perception and behavior, making it valuable to both the Templars and the Assassins. Altaïr's missions eventually focus on tracking the Apple and understanding its significance.
The first game's central Apple is connected to Robert de Sablé, Al Mualim, and the struggle over control of the Holy Land. Its powers are presented through a science-fiction framework rather than conventional magic. The modern-day Animus storyline reveals that the historical artifact has consequences for Desmond Miles as well.
The series contains several other Pieces of Eden, including swords, staffs, and the Shroud. Those objects belong to later stories or have different narrative roles. “Apple of Eden” is the specific name for the spherical artifact associated most strongly with the original game's plot.