One of the plague cauldrons is located in the field of a former farmhouse now referred to as Dalson's Tears.  Well, we shall shed no tears this day as we thwart the Scourge and have them choke on their own fumes!$B$BGo to Dalson's Tears, almost due north of Andorhal, and eliminate the cauldron lord there.  It should have a key like the last one did, allowing you into the cauldron.  Use this bottle to obtain a sample from inside the cauldron; each cauldron has its own distinct brew of toxins it uses.