Once you can access the Hall Of Champions, go to the second floor. Go to the right, and fight through the enemies until you find a breakable wall at the end of the room. Hit the wall, and enter the secret area. It contains a music sheet as well as a breakable floor at the lower right corner. Perform a downward attack to fall through the floor and fight Kratos. Defeat him to get the Grave Digger Shovel. You can then Bring the Grave Digger Shovel to the blacksmith to get the Chaos Armor.