The central artifact is called the Apple of Eden.
In Assassin's Creed lore, Apples of Eden are Pieces of Eden created by the Isu, an ancient technologically advanced civilization. They can influence minds and are connected to the series' recurring conflict over control, freedom, and human choice. Different games place individual Apples in different historical periods and give them different story roles.
The artifact is not a religious object in the ordinary historical sense, although its name deliberately echoes biblical imagery. The series contains other Pieces of Eden, including the Staff of Eden and the Sword of Eden, so “Apple” identifies one type within a larger group. The Apple of Eden also appears in the first game and remains important across multiple later entries.