MUNDAUN

Once you're tasked with visiting grandpas grave, you will find the graveyard door locked. Continue up the hill to the church and look at the girl standing on the higher-up ledge opposite the church entrance. She will throw a paper plane. Follow the trajectory of the plane and pick it up once it has landed inside the little alcove. This will give you the clue to look for the shadow of the church, which points down to the rock to the left of the graveyard door. Return to the graveyard, clear the rock from scrub and pick up the key.