Iron Mask

The Iron Mask was a piece of headwear belonging to the mysterious Man in the Iron Mask.

The man was a prisoner during the reign of Louis XIV of France, and despite the amount of research done, the mask has supposedly never been found. However, in 1794, during the French Revolution, a Cult of the Iron Mask arose. The Assassin Arno Dorian investigated, and was able to recover the keys to a hidden chamber containing the Iron Mask, which he kept.

Reference

 * Assassin's Creed: Unity