Enter the Hammerhall Barrel Chambers at -379, 157, -522 in Butcherblock Mountains. It looks like rubble with pickaxes. Mercenaries are able to go into this instance with you.
Explore the chambers. Some barrels will be destroyable. You may get coin, a snake, an angry bat, etc. Destroy all of the ghosts you can find and a door will open at 197, 17, -89 .