This side quest pops up after you’ve trekked through the Icicle Ridge. Return to Cornelia and talk to the man standing at the base of the stairs that leads towards the Nether Nebula. He will ask you to defeat 8 Goblins so that he can survey their movements. Accept the challenge and make your way to the Watchplains.
Luckily Goblins are easy to find. Run around the plains until you’ve defeated all 8 Goblins and then return to the man. He will thank you and give you 2 Phoenix Pinions as a reward.