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. Well, we shall shed no tears this day as we thwart the Scourge and have them choke on their own fumes!
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 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.