You visit Goron City in the Eldin Canyon region, where you soon discover that something strange is going on.
Quest Location
This quest begins at 1649, 2447, 0381 after you talk to the Goron Elder in Goron Village as part of Eldin Regional Phenomena.
Get Flamebreaker Armor
If you have 700 rupees, or can sell things (such as ore) to earn that much, you can buy the Flamebreaker Armor in the armor shop in Goron City. You will need it soon. You can also buy it at the quest destination if you forgot to buy some in Goron City.
Go Toward the Northern Mine
From the Marakuguc Shrine that overlooks Goron City, take the road north. When you find an area with mine carts, you can attach a fan to the back of the mine cart and place it on the left track, and get in the cart and hit it to start the fan. It will take you to the quest destination. If you would rather go on foot, then keep going north, but when the path splits, stay on the right side. This path will lead you to the quest destination, YunoboCo HQ.
YunoboCo HQ
Talk to the two Goron kids at the quest destination, and agree to try to help. Before you go into the cave, if you don't have Flamebreaker Armor, be sure to go to the other building nearby and purchase some there. You can sell your ores and things if you don't have enough.
Equip the Flamebreaker Armor and go into the cave that the Goron kids were guarding. There will be a cutscene, and you will have to battle Yunobo.
After the Battle
After the battle, there will be a cutscene.
You can now use Yunobo's special ability. Talk to him once to get his attack ready. You will see some glowing dots in a line that point in the direction that Yunobo will go. Point this line at the glowing rocks that are blocking the exit of the cave, then press the button to have Yunobo use his ability. Have him break through the two sets of glowing rocks, and there will be a cutscene.
Visit the Shrine
There is a shrine that you can optionally visit by going east along the road until you find a hill with a large glowing rock in the base. You can have Yunobo use his ability to break the glowing rock, which opens YunoboCo HQ East Cave. Inside, you can defeat some Horriblins and a Fire Like, and use Yunobo's ability to break the rocks surrounding the Isisim Shrine in this cave.
Go to Death Mountain
Your next destination is Death Mountain. The quickest way to get there is to teleport to the shrine at Goron City. Yunobo will rejoin you there.
Go north from there, and after you cross the bridge, go left at the fork in the road. The road will spiral counterclockwise around the hills, then there will be a long bridge over Goron City. Continue along the road from there. You can optionally stand on fallen debris and use Recall on it to float upward to a small sky island. Open the chest on this island to get a Sage's Will, which you can trade at Goddess statues. Then you can attach the rocket to the wing on that island, turn it to face the quest destination, then stand on the wing and hit it to launch it. Yunobo will automatically stay at the front of the wing, where you can have him use his ability to hit any enemies that you might encounter. Just turn the camera to face the direction that you want him to go. Otherwise, if you stay on the road, you will encounter some Fire Chuchus, a Bokoblin, a Rock Like, and some Moblins.
If you stayed on the road, you will reach the Bridge of Eldin. If you go southeast from there, you find some mine carts. There is also a Fan and a Rocket in the debris nearby. Take a cart, place it on the rail so that it is pointing northwest, and attach the Fan to the back, and also attach the rocket to the back. The rocket will disappear after a short time, but the fan will keep going as long as you don't run out of charge.
Get in and hit the fan, and jump out of the cart when you reach another area with mine carts on the right. Put a cart on the rail that goes south, and attach a Fan to the back. Hit the fan, and Yunobo will automatically be ready with his ability, so have him break the glowing rock that is blocking the rail up ahead. When you encounter enemies, be careful not to hit the fan, or you will start going backward on the rail. You can also aim Yunobo at the enemies that you encounter.
Eventually, you reach the end of the rail, at a camp with a Moblin and Bokoblins, so hit the fan to stop the cart. Defeat the Moblin and Bokoblins. Take the spiky metal thing that had been blocking the cart, and attach it to the front of the cart. Then have Yunobo break down any wooden barricades that are still blocking the rail. If you lost your cart, you can get another one and another Fan here.
Make sure that the cart is pointing northeast on the rail, then get in and hit the Fan to start it. Defeat Moblins, Keese, and an Octorok. Eventually, you reach the end of the rail. Hit the fan to stop the cart, and get out.
Visit Another Shrine
Beneath where the rail ends, you can glide down to find Sitsum Shrine. Open its door so you can teleport back here, then use Ascend to get back up to the mine cart rail.
Go Up the Ramp
Go up the nearby ramp and there will be a cutscene. Go up and talk to Yunobo and there will be another cutscene. Afterward, you must battle Moragia. If you have done the quest Camera Work in the Depths, you might want to take a picture of Moragia at the start of the battle.
Go Into the Crater
Glide down into the Death Mountain Crater where Yunobo went. You will end up in the Eldin Canyon Depths.