The chest it is in is hard to spot but run your cursor around and it is there. You won't get the key for it for a while - but make sure NOT to go down the pit in the arena until you go back to the battlefield chest or else it will be a long wait until you can - that pit is one way.
Going through the gate back to where you started the game just go straight through the gate to where you can only turn right or left and the chest is a few feet away.
