What artifact is central to many Assassin's Creed storylines?

The artifact central to many Assassin's Creed storylines is the Apple of Eden.

An Apple of Eden is a Piece of Eden, a class of advanced artifacts created by the Isu, the ancient civilization that existed before humanity's recorded history. Apples can influence minds and produce other powerful effects, making them valuable to both Assassins and Templars.

The first game prominently features an Apple connected to the Templar order and its plans for control. Other games introduce additional Apples or refer to the artifact's broader history.

The Apple is one member of the larger Pieces of Eden mythology. Other examples include the Staffs, Swords, and Shrouds of Eden, so the names should not be treated as interchangeable.