The Saint-Denis Temple key was a key that unlocked an ancient temple built by the First Civilization, located beneath the town of Saint-Denis in France. The artifact needed to be used to gain access to the main hall of the temple. It was used by Suger of Saint-Denis to gain access to the temple at some point in the early 12th century, and during the later half of the 18th century, was kept in a box in the secret vault of King Louis XVI in the Tuileries Palace.
On 10 August 1792, at the height of the French Revolution, the royal family was forced to abandon the Tuileries Palace when it was stormed by an angry mob. Napoleon Bonaparte secretly infiltrated the palace in hopes of obtaining the key, and with the help of the Assassin Arno Dorian, who was seeking the King's correspondence with the comte de Mirabeau, was able to locate the vault and obtain the key.
By August 1794, Napoleon had managed to transfer a small army to Saint-Denis to locate the temple, and put Philippe Rose in charge of its excavation. Arno, unwilling to let the contents of the temple fall into Napoleon's hands, stole the key from Napoleon's chest and used it to open the temple doors. After killing Rose and his men, Arno retrieved the Head of Saint Denis, with an Apple of Eden inside, which he sent to Al Mualim in Egypt for safekeeping.