Need desperate help for Fable 2!?

Ok, so when you beat the game and if you choose the riches option out of the three Theresa gives you and you buy Fairfax Manor, when the bandits attack and you go behind the bookshelf and delve deeper into the castle and underground, you jump off a cliff and swim to shore and it leads you to a room with a door and a little silver ball that you have to shoot. It moves three times and no matter how much you shoot it, it just goes back into it's original place. How do you stop this thing and open the door? It's VERY frustrating, and I don't have my dog anymore so I don't know if there is anything I have to dig up or not. Help!!