Break down a wood barrier to enter the Burstone Quarry, where friendly Gasfellas continue to work even now. Upon entering, a Rattle-tail attacks. This new enemy has the appearance of a white bird, and attacks by burrowing into the ground, causing rocks to rain down on the Kid. Dodge the rocks, as they cannot be blocked, and strike at the Rattle-tail with a melee weapon, even if it is still burrowing. This will force it out of its hiding spot, allowing you to strike while it flails until its defeat. Further along the path is a switch that will move some pillars in the way, forming a path. Another switch ahead does the same. A third switch frees a large Lunkhead; feel free to attack it while it is trapped if the Kid is holding a weapon with an armor-piercing upgrade, then activate the switch to continue forward.
Past two more switches is the Arsenal. Before activating another switch, take the exposed western path to find Something Stringy. Backtrack and activate the switch to open the path forward. Keep activating switches to proceed, until you come across an arena where a massive Lunkhead attacks. Dodge its leaps and strike its back when it lands just as you would any other Lunkhead. After defeating the enemies, look around the top of the area (where the Lunkhead first appeared) to find Something Coarse. Multiple gates should open after the battle, revealing a new path where a familiar face can be found. Interact with him and then activate the switch to raise the cage guarding the Shard. Pick up the Shard and go through another gate to find the Skyway that leads back to the Bastion.
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Back at the Bastion, fight off some Ura Warriors as you move along the path. The path will be blocked at one point, requiring the Kid to take an alternate path. This path will then branch three ways; head south to find a grassy patch where a memento, the Harp Guitar, sits. Break down the fence to get to the Skyway that sends the Kid back to the hub, where any pets he has collected will be assisting in his fight against the Ura. Defeat them all to trigger a scene.