Ark of the Covenant
The Ark of the Covenant was hidden under Solomon's Temple and housed a Piece of Eden, the Apple of Eden.
The Master Assassin Altaïr Ibn-La'Ahad, along with the brothers Malik and Kadar A-Sayf were sent to the temple to retrieve the ark in the opening of Assassin's Creed. During the mission to retrieve it, Altaïr broke all three tenets of the Creed. The first when he killed an innocent old man so as not to alert anyone of their position, then the second and third tenets while foolishly trying to kill Robert de Sable, the Grand Master of the Knights Templar, who happened to be there with some of his Templar guards to retrieve the Piece of Eden himself. This failure nearly caused the Assassins everything, as Kadar was killed, Malik was severely injured, and Templar forces were compelled to attack the Assassin fortress of Masyaf.