The powerful spherical artifact in Assassin's Creed is the Apple of Eden.
An Apple of Eden is a Piece of Eden, a group of advanced artifacts created by the Isu, an ancient civilization that existed before recorded human history in the series' mythology. The Apples are associated with abilities such as manipulating minds, creating illusions, and influencing human behavior.
The artifacts are important because both the Assassins and the Templars seek them. The Templars generally want to use Pieces of Eden to impose order and control, while the Assassins oppose that goal and defend human freedom. This conflict connects many different historical storylines.
The series includes multiple Apples rather than one ordinary, unique fruit. Other Pieces of Eden include swords, staffs, and shrouds. The word “Apple” refers to the artifact's shape and traditional name within the fiction, not to a normal edible apple.