Destroy 20 Scourge of any type at Talramas and the Festering Pools. Then, return to Bixie Wrenchshanker just to the south of The Dens of the Dying.
20x Talramas Scourge Destroyed
If the cultists up in those caves survived the crash, then that fallen necropolis to the west can only be assumed to still be filled with Scourge. There're probably lots of them out in front of it in the pools, too!
While you're at Talramas, [class], do you think you could dispatch all of the Lich King's flunkies that you find there?