In Fable 3 for the Xbox 360, there is a secret gold key and chest hidden in the sanctuary.
Once you are the King or Queen of Albion, go to the Sanctuary and enter the Tresure Room. Look up and you’ll see a gold key that you can’t reach. In order to reach the key you’ll need to put about 5 million gold pieces into your treasury to climb on.
Once you have the key, the chest it opens is ironically now beneath your 5 million in coins. Transfer all of the gold to the Albion Treasury, which can be done in 1 million coin increments. Opening the chest will give you a random legendary weapon.