Gahrron's Withering houses the last cauldron that we need to acquire access to here in the west. Once we have a sample of the plague from that cauldron, we will be able to introduce our reagents into the environment through four separate vessels! The Scourge will be hard pressed to keep us out of all four, even with the power of Kel'Thuzad behind them.
Congratulations <name>, you've unlocked the last cauldron for us! Apothecary Dithers will soon know what this cauldron requires as far as reagents needed for the mix.
As for you, I am sure that High Executor Derrington will want to thank you personally for the assistance you've given us. Without you, our struggle against the Scourge would have been nearly insurmountable. I thank you, mighty <class>.