This can be achieved in Chapter 6. Right at the end of the game you have to make a choice to get one of the games 2 endings. You want to pick the option where you leave and forget about Harry.
Your character will then leave the house and play the music box. Walk down the long path that appears until you get to some stone rings (see pic)
There is an invisible path to the left follow it until you come to a rock with the key on it. Simply interact with it for the achievement!