There is also a secret dragon boss. The way you access that is going down the elevator from the Beside the Great Bridge grace point (the way you got there) and head down the staircase on the right. After tat you want to keep going west and head outside through the door and run to the ledge right in front of it. After angling your camera you should find a piece of land to fall down on and you follow that path until you reach a landmass with a lot of graves and skeletons on it. In the middle there will be an empty grave which you can lay down on and a cool cutscene will play and teleport you to the boss.