After you complete the Turtle Rock dungeon, you have all eight Siren Instruments. You can go to the Wind Fish's Egg in the middle of the Tal Tal Mountains and use the Ocarina to play the Ballad of the Wind Fish. This will open the egg.
Learn the Maze Route
Before you go to the Wind Fish's Egg, go back to Mabe Village and go to the library. If you have the Magnifying Glass, read the bottom-right book in the library, then write down the path that is in the book. The path is random, so you will have to write it down. If you do not have the Magnifying Glass, read the Trading Sequence page to find out how to get it.
Go to the Wind Fish's Egg
After you have written down the path, go to the Wind Fish's Egg in the Tal Tal mountains. One way to get there is to go east from Mabe Village and use the warp spot there. Keep going into the Warp Spots until you reach the area with three waterfalls, then go west until you see some stairs. Go up those stairs and keep going west, then go past one set of stairs and go up the second set of stairs to reach the Wind Fish's Egg.
Go up to the egg and play the Ballad of the Wind Fish on the Ocarina. After a cut scene, a hole will open in the egg. The owl will fly down and tell you to go into the egg. After that, the owl will stay there. Go in.
Go Through the Maze
Inside the egg, go north until you fall into a pit. You will land in the maze.
Use the directions that you found in the book in the Mabe Village Library. If you take the correct path, you will reach a big hole in the ground. Fall in.
Nightmare of Giant Bot
After you fall in, the nightmares will speak to you, and then you have to battle them. First, you fight the Nightmare of Giant Bot. Wait for it to start bouncing around, then sprinkle Magic Powder on it. Do this three times to make it change form.
Nightmare of Agahnim
The Giant Bot transforms into Agahnim. The Nightmare of Agahnim will throw energy orbs at you. Use your sword to knock the round energy orbs back at Agahnim, but stay away from the X-shaped orbs. Be sure to stay in a line with the X-shaped orb, because it shoots out diagonally after it hits the wall. After you have hit Agahnim enough times, it spins around and transforms into its next form.
Nightmare of Moldorm
Hit Moldorm's tail to damage it. After each hit, it moves more quickly for a while, then slows down again.
Nightmare of Ganon
After you defeat Moldorm, it transforms into Ganon. Use Spin Attacks to damage Ganon. Ganon will create flames around himself, then the flames become Fire Keese and fly at you. Move around the room to avoid the Fire Keese, then charge up a Spin Attack and sidestep the trident that Ganon throws, then hit Ganon with the Spin Attack while the Trident is coming back. Then move out of the way when Ganon floats to a different part of the room.
Nightmare of Lanmola
After you have hit Ganon enough times, Ganon transforms into the Nightmare of Lanmola. Hit it with the Magic Rod to kill it in one shot.
Nightmare of Dethl
Avoid Dethl's spinning arms (use the Roc's Feather to jump over them), and hit Dethl in the eye with the Boomerang when it opens its eye. This will kill it instantly. Otherwise, you have to hit it with 15 arrows in the eye to kill it.
After you kill Dethl, you have won the game. The ending of the game has an extra part if you did not die during your adventure. Congratulations! Enjoy the ending!