When you have gained access to the Hall of Champions go to the second floor. Then go to the right and fight through the enemies until you come to a breakable wall at the end of the room. Hit the wall and enter the secret area. There you will discover a music sheet and a breakable floor at the lower right corner. If you perform a downward attack you will fall through the floor and face Kratos. Defeat him to get the Grave Digger Shovel which you can then take to the blacksmith to get the Chaos Armour.