Throne and Liberty Infiltration Operation Starts
Chapter 13: Shadow in the Black Mine Exploration Codex Entry

You've discovered the suspicious location listed in Bonia Naru's order. This may be where Clay is being held. Search it thoroughly.

Objectives

Examine the Minecart at Eastern Obsidian Mine
Follow the rail at the Underground Mine to find evidence of the abyssal bitumen bottle
Drop the item hanging in the air to cause a commotion
Find a way to drop the item hanging on the ceiling
Defeat Black Anvil Dwarf Guards

Rewards

Precious Armor Growthstone