Down the Drain...  

Now that you've found out Anashti Sul is responsible for the undead, Prophet Bainyn Fairwind wishes you to head into Edgewater Drains to stop a follower of Bertoxxulous from spreading a plague. This could be a little difficult at the quest's intended level. May want to consider taking 1 or 2 people for some help.

Once you're in The Serpent Sewer, take the level 1 entrance to Edgewater Drains. When you finally get in there, head to 103.06, 32.50, 22.77 Copy and kill an Agent of Bertoxxulous. He'll update your quest and now you have to return to Bainyn as you feel a bit unwell. Well really... what did you expect from a disease-carrying Ratonga? The prophet tells you that he needs a Distilled Oaken Root vial, which you can buy from Viddle Sprocketbender in Zek.

Head there now. You'll find Viddle sitting on the docks, so he's not hard to miss. Buy the Distilled Oaken Root Vial for 6 silver. Now, head back to the city and buy a Superb Noxious Remedy from the broker... or find a friendly alchemist to make one for you. Once you have both of those, go back to Bainyn to complete the quest and receive your choice of reward: Polished Symbol of the Prime Healer or Silvered Symbol of the Prime Healer

Note: The noxious remedy step may not update when you pull one from bank or guild bank. If so - find a merchant, sell it and buy it back to update the quest.