Chronos: Before the Ashes

After you defeat the Pan Guardian, you will find yourself back in the first world. A new path has opened up behind you (metal door now broken open). Follow this into a garden like area and defeat the enemies there. Watch out for the little guy with the stone cannon, shields up when you approach him. Once cleared, look for a stairway down - there is a stone arch with a single torch you must go through to reach these stairs. Follow the stairs down to collect your new magic stone and this achievement.