In the Assassin's Creed series, the powerful Isu artifact often called the Apple is the Apple of Eden.
The Apple of Eden is one of the series' Pieces of Eden, advanced artifacts created by the Isu, an ancient civilization that existed before humanity's recorded history. Its abilities commonly include influencing minds, generating illusions, and projecting energy, though the precise effects vary across games and storylines.
The artifact connects the modern-day narrative with historical adventures. Characters such as Altaïr, Ezio, and other Assassins encounter Apples while opposing Templars who seek to use Pieces of Eden to impose order and control. The Apple therefore represents both a technological mystery and a recurring source of conflict.
The series includes other Pieces of Eden, including the Staff and the Sword. The Spear of Leonidas is also an Isu-related weapon, but it is not the artifact generally known as the Apple.