One of the plague cauldrons is located in the field of a former farmhouse now referred to as Dalson's Tears.  No tears will be shed this day, $c; we must strike at this cauldron and gain access to it!$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 inside the cauldron's innards.  Use this bottle to obtain a sample from inside the cauldron; each cauldron has its own distinct brew of toxins it uses.