After you grab the Blue orb on the underground temple you will see a chess to the right on the hallway next to the blue orb altar, make sure to open it, on the chess choose left, right, left, and it should open, you will get rope ladder, later on, you will cross a bridge and find yourself in a square area with a bunch of Oni's that keep spawning, keep going to your left and you will find a door, go through it and in the next area you find a courtyard with a big hole on a wall, get close to that hole and press option button, and use Rope ladder on it, now you can go down and fight Osric.