Fyrus

Twilit Igniter: Fyrus (覚醒火炎獣 マグドフレイモス, Awoken Flame Beast: Magudofureimosu) is Darbus's possessed form that he assumed once the Fused Shadow took over his body. Due to his ferocity and inability to control himself from the mutation as well as causing destruction in the mines, the Goron elders locked him away deep inside them to prevent him from causing further damage. As with most bosses in the Zelda series, Link must first make Fyrus vulnerable and then attack his critical point repeatedly to defeat him. To do this, he will need the Hero's Bow and the Iron Boots.

First, Link must attack the glowing jewel on his head with an arrow to momentarily stun him. He must then equip the Iron Boots and grab the end of one of the chains attached to his legs and pull. This will cause Fyrus to fall over. Afterward, the jewel on his forehead (his critical point) will be vulnerable and Link can attack with his sword. When Fyrus finally recovers, he will emit a wave of energy that can damage Link, so be sure to get as far away from him as possible after attacking.

Once Fyrus is defeated, the twilight matter from his altered form fuses into one piece, creating the second Fused Shadow. Link is able to look upon a confused Darbus as the Goron returns to his normal self, completely unaware of what had happened. Link is also awarded a Heart Container for his efforts.

Trivia

 * Fyrus was the first boss ever revealed for Twilight Princess. He was present in the first trailer for the game, and is also the only boss fight in the game without a second round.
 * Fyrus is similar in appearance to a Balrog from The Lord of the Rings. In addition to this, the jewel on his head closely resembles the Eye of Sauron at one point.