After his victory in the Tower of the Gods, Link travels to a mysterious place in the depths of the ocean.
Move the Triangles
Ignore the black and white frozen Moblins and Darknuts for now, and go down to where there are triangular blocks and a Triforce symbol. Your boat will talk to you with A.
You can push and pull the triangular columns. If you push on the left side, the column will rotate to the right, and vice versa. Similarly, if you pull on the left side, the column will rotate to the left, and vice versa. There are two triangular columns that are pushed up against each other, and one of them is up against a raised grey triangle on the ground. Go to the one that is not pushed up against the raised grey triangle, and pull on it. This allows you to pull on the other one. Continue to pull and push the blocks around until you have placed one on each of the light grey triangles on the ground. When you have done it correctly, the hero statue will move to reveal stairs. Go down.
Get the Sword
Go left or right down the stairs and make your way to the bottom of the room, then go up the short flight of stairs where a light is shining from above. Walk up to the beam of light to take the sword. Watch the cutscene.
Feel free to look at the stained glass windows in this room. They should look familiar if you have played The Legend of Zelda: Ocarina of Time.
Defeat all of the Monsters
Before you can continue, you have to go back upstairs and defeat all of the Moblins and Darknuts. Run away from the first ones that attack, and try to reach the upper walkway, where there are fewer enemies. Since Moblins don't have armor, you can try to pick them off with arrows, but you might lose out on some spoils because you will be too far away to reach them in time. Try to lure the enemies away from each other so you can fight them separately. Use the Boomerang on Moblins, and on Darknuts if you can remove their helmets. If there are too many enemies around you, just run away until they stop chasing you, then try to fight enemies one at a time.
These enemies often drop glowing spheres. Be sure to collect all of the spoils. This is a rare opportunity to gather a large number of spoils at once.
Go Back Above the Water
After you have beaten all of the monsters, the doors will no longer be electrified. There is a semi-invisible wall if you go through the back door, and some scenery to look at. When you're ready to move on, go up the stairs in the castle and go out through the front door, which is between the paintings. Get back in your boat and cruise into the glowing spot to go back above the water. You can now learn a new song: the Ballad of Gales.