This is a two part achievement. The first part is a letter that is picked up when you first make your way to the village on the circular wall. The bottom most doorway on the right has a children's room with a letter. Interact and pick up this letter.
Proceed past the city and once above look to the left hand side of the wall in front of you. You will see some railings and a pasture with a few goats. Climb your way over to it, proceed into the cave and watch the cutscenes. To complete the achievement you need to walk up the stairs and interact with the shepherd again.