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, <name>.
Gahrron's Withering is due northeast of our position here, 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 Light protect you in this dangerous mission, <class>.
Cauldron Lord Soulwrath's key disintegrates as it touches the lock on the access panel at the base of the cauldron's platform. A slight clanking noise is heard within, and the access panel swings open to reveal more viscous toxins within the cauldron's framework.
There is a small spigot that the bottle fits well enough under, allowing you to draw a sample for High Priestess MacDonnell back at Chillwind Point.