Fyrus

Fyrus is Darbus' 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 the mines so he won't cause damage in the mines. As with most bosses in the Zelda series, you must first make Fyrus vulnerable and then attack his critical point repeatedly to defeat him. To do this, you will need your Hero's Bow and the Iron Boots.

First, attack the glowing jewel on his head with an arrow. This will temporarily distract him with pain and give you the time you need to knock him over. 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. Quickly run to the front of the large creature and repeatedly attack the jewel on his forehead with your sword (the Critical Point).

When he gets up he will emit fire and you need to be far away from him when he does. Repeat the process until he is defeated. Once Fyrus is defeated, however, 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, unable to realize what had happened to him thus far. Link is also awarded a Heart Container for his efforts.

Trivia
Fyrus was the first ever boss revealed for Twilight Princess. He was present in the first trailer for the game. He is also the only boss in the game without a second round. He has a similar appearance to a Balrog from The Lord of the Rings.

In addition to this, the jewel on Fyrus' head closely resembles the Eye of Sauron at one point.