This chest appears in the secret fairy cave behind the waterfall in the north of the overworld.
You will need to find all 20 fairy chests before you are able to obtain this secret achievement.
I used the guide below to help me with getting all 20 fairy chests, use page 48 to see which ones you're missing.
https://techraptor.net/gaming/guides/tunic-secret-fairy-guid...For people having trouble with number 8:
Make sure it's night by sleeping, go to the teleporter and use the pad that you have to jump over too on the right, this will get you to the forest area.
Go to the start of the area, remember when you first ever came here, you would have dropped down some ladders and couldn't go back that way?
Use your ability to jump just outside of this position to climp to your top right, there is a chest that you may have already opened up there. Do the D-Pad combination and it's yours.