There's a dragon priest down there. A named one. After you jump off the ridge and into the area with the giant red door and the bloodskal blade, there's no turning back. The key to the door's puzzle is that you have to aim the beam of the sword (with power attack) to the same direction of the lines of red energy. If they're horizontal, flick your joystick to the left or right when you power attack and it will sideways slash, throwing a horizontal beam. Power attack normally for a vertical beam.