Go to Dalson's Tears in Western Plaguelands to locate and defeat the Cauldron Lord present there, and use its key to gain access to the cauldron. You must have the Empty Dalson's Tears Bottle with you to secure a sample of the poisons used inside the cauldron.
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, [class]; we must strike at this cauldron and gain access to it!
Go 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.