Go to Felstone Field in Western Plaguelands to locate and defeat the Cauldron Lord present there. It may have a key that will allow access to the cauldron. You must have the Empty Felstone Field Bottle with you to secure a sample of the poisons used inside the cauldron.
Of the four cauldrons in the Western Plaguelands, the one at Felstone Field may be the easiest one to gain access to. The Scourge have greener troops positioned here, though they should not be underestimated.
Go to Felstone Field, due east of here and northwest of Andorhal, and engage the cauldron lord there. On it is a key that will unlock access to the cauldron. When unlocked, use this bottle to get a sample of the cauldron's toxins; we'll need it for study in developing a counter agent.
