Complete the Disloyal Denizens World Quest, the Spires of Ascension Dungeon, or the Necrotic Wake Dungeon in Bastion.
Defeat the powerful foes of Bastion (1)
We consider the denizens of Bastion to be allies of a sort. Without their service, we would have no souls, and without souls, we could not raise armies, and without armies, we could not smash our fists against one another in the glorious theatre of battle!
<Grandmaster Volo laughs and smashes his weapon around.>
We must aid them against their foes, or else the arena will sit barren, and we can't have that!
Find their greatest enemies and turn their bones into golden dust.