The Choir is a hidden area above the Healing Church Workshop. To access it, first go to the Chapel lamp in Yahar'gul, Unseen Village. Then go up the long staircase with the laser of doom, and then look for a place to the left that you can drop off. Go into the nearby doorway and then drop off again just ahead and to your right. You should now be in a small cell with a corpse and an item. Pick up the item, which is a key.
Now go to the Cathedral Ward lamp, and go in the door on the right side and then up the elevator. Follow the path to the Healing Church Workshop, and then go up to the top of it. At the very top is a door you can unlock with the key that you got in the Unseen Village. Open the door and enter to unlock this trophy. You can then continue on into this area to earn the "Celestial Emissary" trophy. See the video guide below: