The Armory Key is the key to unlock the Armory Room which you will find more items and information here.
How to Get the Armory Key
On the fire place, light up all the chandeliers in the room there will be four of them. When you’re done, you’ll see the glass eye in the floor pick it up and jump up the ladder to the top of the bookshelf and there will be a tiger’s head mounted on the wall. Use the Glass Eye from your backpack and the tiger’s mouth will open, revealing the Armory Key.