Fyrus

Twilit Igniter: Fyrus is Darbus's possessed form that he assumes after the Fused Shadow takes 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 lock him away deep inside the Goron Mines 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 needs the Hero's Bow and the Iron Boots.

Strategy
The first step for Link is to 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 causes Fyrus to fall over. Afterward, the jewel on his forehead (his Weak Point) becomes vulnerable and Link can attack with his sword. When Fyrus finally recovers, he proceeds to emit a wave of fire that can damage Link, who must be sure to get as far away from him as possible after attacking.

An alternate way to defeat him is to shoot him in the eye-gem and then wait until he calms down, then shoot him again, and repeat. Eventually he falls down and he can be instantly killed with the finishing blow. This strategy takes longer and uses more arrows.

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 has just happened. Link is also awarded with 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.
 * 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.
 * The strange, boar-like helm on Fyrus's head resembles that of Ganon's face.
 * Fyrus is the only boss in Twilight Princess to have only one phase in his battle. All of the other bosses have two or more.
 * Fyrus is also the only Fused Shadow boss that does not completely dematerialize into twilight matter after defeat. This may be because he is a sentient being, whilst almost all the other bosses were versions of normal enemies found in the game which became mutated by the power of the fused Shadow (and, later, the Mirror Shards).
 * Fyrus' name is a portmanteau of Fire and Darbus, the name of Fyrus' Original form.