Once you are the Ruler of Albion, enter the treasure room in The Sanctuary (your pause menu) and look up and you should be able to spot a Gold Key. To reach it, you'll need to make some 5,000,000 Gold. The Scrooge McDuck-like pile will lift you to the key. One way to do this is to buy shops early and often to start turning a profit. High-priced shops turn higher profits. You can get a cool million from the Sunset House Demon Door as well. A special chest to match this key lies at the very bottom of your Gold hoard. To reach it once you have the key, transfer ALL of your funds to the Albion Treasury. This will take 5 transactions of 1 million Gold. Now, return to the Sanctuary and open the chest for a Legendary Weapon.