This quest was marked obsolete by Blizzard and cannot be obtained or completed.
Unlock the secrets of the Temple of Atal'Hakkar to release Atal'alarion and recover the Putrid Vine from his flesh. Return to Mehlar at the Bulwark when you have done this.
If you wish to turn these stones into a weapon to fight the Scourge, you must acquire something equally vile to fortify them.
An ancient evil lies hidden at the bottom of the Temple of Atal'Hakkar in the Swamp of Sorrows. Release the creature there and destroy him, taking only a small part of his body back to Mehlar Dawnblade. My guards will deliver the scourgestone fragments to Mehlar.