Go to the Writhing Haunt 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 Writhing Haunt Bottle with you to secure a sample of the poisons used inside the cauldron.
Two cauldrons have been successfully opened; while you will be able to access them indefinitely, we still want to open up the other two that remain. This will provide all of us with more options to introduce the counter agent!
This time, you'll be hitting the Writhing Haunt's cauldron. It is almost due east of Andorhal, relatively close to the ruined city walls; again, eliminate the cauldron lord present there for its key. Use this bottle to obtain the sample from the cauldron.
Good hunting, <name>!
The cauldron lord's key unlocks the access panel to this cauldron and then disappears, much like the other keys have for the previous two cauldrons you've dealt of. A putrid smelling substance churns violently beneath the platform of the cauldron.
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.