"Apparently there are a couple of stragglers who have decided they won't accept my rule. The hard-headed ex-Avalon citizens Griswold and Raynor have occupied the old Marketplace. I don't care about Raynor, but Griswold has socketing talents that could potentially aid the Resistance. Go to the marketplace, and kill Griswold."
Exit the throne room and head across the small bridge and down the stairs to the marketplace. As soon as you enter the market, both Griswold and Raynor will attack you, but you are not required to kill Raynor. Once you have killed Griswold, return to Emperor Xanthar.