This is a
missable achievement. When you reach the second half of the game, and you can choose where to go next, make sure you do this
before completing the quest you get in your village. Don't return the Astral Tea Leaves to the elder until you have this achievement.
To get it, warp to the Autumn Hills. Head up to the surface, then right, then down here to the Catacombs:
Now head right to the middle of the Catacombs, and down to the Dark Cave:
You get the achievement for completing it in the dark. It's tough, but it's just three screens.
Here's a map I made of the whole area, showing how to navigate it:
And here's a video of someone completing it with light:
I recommend watching the video with the map in front of you, then playing the cave yourself with the same map. If you'd like to see it in the dark, that's here:
With the map and videos, this will take a few tries, but it's perfectly doable. Good luck!