Twilight Igniter: Fyrus is the form Darbus assumed when he touched the Fused Shadow. He was unable to control his new form and so the Goron Elders locked him away deep inside the Goron Mines, to prevent him from destroying more of their resources. He is the final boss in the Goron Mines dungeon, which Link must beat in order to recover the second Fused Shadow.
 Boss Strategy
Fyrus is an easy boss, as long as you are good with the Bow. The first step to defeating him is to bring him to Link's level, by shooting the glowing jewel on his forehead with an arrow. This will momentarily stun him, allowing Link enough to bring the beast down. First, he must equip the Iron Boots, so he doesn't wind up being dragged around as the boss stumbles around the room. Next, grab the ends of one of the chains trailing behind Fyrus. Thanks to the added weight of the Iron Boots, Fyrus can no longer run freely and will trip if Link begins to walk backwards. As soon as he hits the ground, run over and begin slashing away at the jewel like a madman. Repeat two or three more times and he should go down for good.
When the Twilight enshrouding Darbus finally dissipates, Link is rewarded with the second Fused Shadow, as well as a Heart Container.