The Dark Prophecy

Story related, can't be missed.

Once in the village, talk to the witch, exhausting all dialogue to get the key. Use the key on the witches house, and then go inside.

Once inside, use the chest at the bottom, and select the following images going clockwise from the top. Cloud, Rain, Puddle, Puddle With Steam.

The chest, along with the achievement, will unlock.

You get this achievement after unlocking the chest.
Change the right button to train using the up or down arrow.
Change the bottom to a puddle.
Change the left one to show the puddle evaporating.
After you change the last one, the round area between the images will fill in blue, when it completes the circle, the chest opens.
Congrats on your achievement!