Requirements: Black Chest Key. Level 66+
The next NPC can be found at Windmill Way, just outside the Glittering Windmill in the Pukilipo continent. Find the Gate at G-6 and use your Black Key to enter, then hop down the well. Inside you'll find a lone NPC.
He asks you to go defeat a Scarab King in Windmill Hill or the Gobbie Desert. Scarab Kings are the nasty looking green and yellow beetles with large blue tails. I found a plethora of them at F-3 of Windmill Way.
Once thats done return to your starting NPC in the well and congratulations!