This quest is not available in game.
Bring the Head of Tel'athion to Aqueous in the Hidden Reef at Bloodmyst Isle.
I have determined where the corruption is coming from. It is the by-product of some demented research and you're going to put an end to it!
Just to the southwest of the Foul Pool is a small camp. There you will find the 'research' of Tel'athion the Impure. This is where he produces the filth that has been seeping into the island's water.
Use the pure water you obtained from Ashenvale to destroy his cache of barrels. That will be sure to bring him out.
We will have an end to this!