Go to the room after the bonfire and climb the ladder (just circle the area, you'll find it) where you will meet two headless enemies. Kill them near the golem to make it turn the goblet, go back down the ladder and their is a elevator. After this go back to the room with headless enemies and follow the bath to a branching staircase. Go to the hallway opposite of the staircase and open the door. Just follow the path till you get to a room with three chests. You will get the Kings key. Go back to the bonfire and go up the elevator. Open the door using the Kings key and this will lead you to the looking glass knight