This is one of two ending achievements. Don't sweat about a 2nd playthrough as you can just replay the last boss. After the final battle you will need to press
in front of either the left or right mural. This achievement is for the Left mural. Once unlocked simply reload your save and go over to the eagle shrine on the right, then to the far right side for two shrines, the left of which is the final battle. Replay the final battle and choose the opposite mural for the other achievement.