First you need a Devourer Egg, you can either do The Egg Hunter quest which gives you 2 devourer eggs as a reward, or you can get an egg from collectors Brownlow or Rownan (in exchange for 5 Skale Fins or 3 Dull Carapaces, respectively). Then take that egg back to Pitney. After you've handed over the egg, follow him to the fields just north of Ashford Abbey and kill the Worm Queen that comes out of the ground. Finally talk to Pitney again and claim your reward.