Before the start of the game a group of Gorons went into the mines to investigate the invasion of enemies. The Gorons came across one of the Fused Shadows, and when Darbus touched the Fused Shadow, he was corrupted by the dark powers. Darbus transformed into a giant and violent monster. The other Gorons were forced to chain him up and lock him away in the mines.
Link gained the trust of the Goron elders by winning a wrestling match, so he was allowed to venture into the mines. Link manages to free Darbus of the Fused Shadow's control. When Darbus wakes up, he has lost all of his memories and the Goron's are to proud to tell him he was saved by a mere human, so he doesn't show any gratitude untill later in the game.
When Link finally obtains the key to Fyrus' room, he enters an dark bluish-green chamber. The door locks behind him, as usual. In the middle of the room is Fyrus, arms and legs chained to the ceiling and floor. As Link approaches the beast, a crystal on his forehead starts to glow and Fyrus awakens. He notices the chains and sets his body ablaze, which weakens the chains, allowing him to break free. Fyrus starts to rampage, trying to kill Link. His main attacka consist of swinging his chains around and creating walls of fire.
To defeat Fyrus, Link must first shoot the crystal on his forehead with the Hero's Bow before running to the back of him and grabbing one of the chains on his feet. If Link wears the Iron Boots and tugs the chains, it causes Fyrus to fall to the ground, allowing Link to slash at the crystal on his forehead with his sword. After repeating this several times, Fyrus turns back to his Goron form.