Go to Gahrron's Withering 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 Gahrron's Withering Bottle with you to secure a sample of the poisons used inside the cauldron.
The last cauldron that remains is located on a farm now called Gahrron's Withering. It is guarded by some of the Scourge's tougher minions - specifically, wraiths and ghosts. They are not to be trifled with, [player].
Gahrron's Withering is far to the east of the Bulwark, close to where the Eastern Plaguelands begin; again, eliminate the cauldron lord present there for its key. Here is your empty sample bottle; bring it to me filled. May the shadows cloak you in safety for this dangerous mission, [class].