After you get all eight Essences of Nature, go to the Maku Tree in Horon Village. He gives you a Huge Maku Seed, which has the power to penetrate evil forces. This allows you to enter Onox's Castle.
Go to the Castle
To reach Onox's Castle, warp to North Horon. Go south, west, north, west, go across the bridge and go north twice, then go west through the hollow log. Go north, kill the Ball and Chain soldier, then go west, and north. The maku seed uses the essences to dispel the evil power protecting the castle. Then a cloaked figure appears and turns into Twinrova, who says that they will revive the Evil King. The Maku Tree urges you to defeat Onox and rescue Din.
Go north until you reach the entrance to the castle. Go inside.
How to Heal Up in Onox's Castle
Whenever you go west or east in this place, you reach a room that has a fairy, but these rooms also have tiles that throw themselves at you. Try standing in the doorway and using the magical boomerang to grab the fairy and prevent the door from locking you in.
Go north from the entrance, killing all of the enemies in each room and continuing north. If you don't kill all of the monsters in a room, going north just sends you back to the room you just left. You will face Wizzrobes, then Darknuts and Wallmasters, and then you will have to once again fight the mini-boss Facade.
To defeat Facade, place bombs in the center of the room when Facade appears. (It's only vulnerable to bombs.) Keep moving around so you can avoid the beetles, fireballs, and holes in the floor that Facade creates.
Go north from Facade's room. Break the pots on the sides of the room to restore your items.
Go north to face Demon General Onox.
Demon General Onox
Onox is only vulnerable to spin attacks. Onox swings his mace around much like a Ball and Chain soldier. The safest direction from which to spin attack him is the bottom left. He will sometimes throw the spiked ball at you. Try jumping when this happens. Also, he can make boulders fall from the ceiling, so move around a lot and avoid shadows. In addition, he sometimes throws whirlwinds at you. Try to jump over those.
Eventually Onox will use the crystal that Din is trapped in as a shield. If you hit the crystal with your sword, you get electrocuted. Use the Rod of Seasons to knock the crystal out of the way whenever it's in the way, and continue spin attacking Onox as before.
Eventually the floor will collapse. Fall down and Onox will appear, and will turn into Dark Dragon Onox.
Dark Dragon Onox
The dragon will smash his hands toward you. Avoid the hands, then jump onto the back of a hand and wait for Onox to pull it back up. Jump from the hand and hit Onox's face with a spin attack. Try to wait until Onox is flying in the direction of the hand that you are on. For example, if you are on the right hand, wait for Onox to fly toward the right. Otherwise, Onox's face might go out of reach when you try to jump and hit it.
Sometimes Onox will breathe blue fire. These flames move slowly, but the flames are tall, so try to sidestep them in such a way that you don't have to jump over them. Sometimes he shoots red fireballs. Try using Pegasus Seeds to stay out of the way of those. In addition, sometimes Onox swipes his claw sideways along the ground. Try to move away from Onox if he seems to be moving toward the side of the screen. That way you can wait on the far side of the screen, which should be safe because Onox doesn't swipe all the way across.
After you have hit Onox's face enough times, he will be gone for good. Walk up to Din's crystal and she will break free. Watch the ending, then when the "The End" screen appears, press A and then make a note of the Secret of Labrynna. Use this when creating a new Oracle of Ages game so you can get rid of Twinrova!