"What's the matter, Guardian? Does Avalon look a bit different from what you're used to? You had better get used to it, because my Master and I have taken over. Now, while you're here, you may as well make yourself useful. Let me fill you in. After CluDathoon activated the Camlann Device and attacked Upper Avalon, many of the city's residents fled into Lower Avalon. The Vampire known as Demona emerged from the Sewers, and began slaughtering everyone. Some residents were bitten, and turned into Infected. They are such a nuisance. How am I going to move my own soldiers into Lower Avalon, with those brainless zombies attacking anything that moves? Go to Lower Avalon, and kill at least 10 Infected."
Leave Xanthar's throne room and locate and descend the stairs to Lower Avalon. The Infected are located throughout the town. Once you have killed 10 Infected, return to Emperor Xanthar.
Note: When you type /quest, it will not inform you of how many Infected you have killed so far, but it will still notify you when you have killed all 10.