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 there to retrieve it in the opening of Assassin's Creed. During the mission Altaïr broke all three Tenets of the Creed; one 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 it himself. This failure nearly caused the Assassins everything, as Kadar was killed, Malik severely injured, and it brought the Templar forces into the Assassin fortress of Masyaf.