Open the gates covering the fireplace and crawl through the gap. Pick up the Crimson Glass from the desk, then head out through the doors. Interact with the door to your left to trigger a scene, then interact with your hands to get down. Head down the stairs through another door into the Hall of the Four. Go back to the entrance hall, and to the door on your left.
Aim your gun at towards the pillar, and shoot down the Crystal Fragment. Once inside, go to the elevator room to your right.
