The recurring powerful relic in Assassin’s Creed is the Apple of Eden.
An Apple of Eden is one of the Pieces of Eden, advanced artifacts created by the Isu, an ancient civilization that existed before recorded human history in the series’ mythology. The Apples are associated with powerful effects, including influencing human minds and projecting illusions or energy.
Different games place individual Apples in different historical periods and connect them to figures such as Altaïr, Ezio, and Rodrigo Borgia. Their presence helps link otherwise separate historical stories to the wider science-fiction framework of the franchise.
The Apple is not the only Piece of Eden. The series also features the Staff of Hermes, Swords of Eden, and the Shroud of Eden. Those artifacts have different forms and functions, so “Piece of Eden” is the category, while “Apple of Eden” identifies this particular type.