Ark of the Covenant

The ark of the Covenant was hidden under Solomon's Temple and housed a Piece of Eden. Altair was sent there to retrieve it in the opening of Assassin's Creed. On the mission he broke the three tenants of the Creed which caused him to get thrown out of the battle when he encountered Robert de Sable there as well. This nearly caused the Assassins to not get the Piece of Eden and demoted him to from a Master Assassin to the lowest rank in the order.