Gahrron's Withering houses the last cauldron here in the east that we need access to. Once we have a sample of the plague it spews forth, 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! Alchemist Arbington will soon know what this cauldron requires as far as reagents needed for the mix.
As for you, I am sure that Commander Valorfist 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. Thank you, hero!