Slay 6 Mannoroc Lashers, 6 Felguards and 6 Searing Infernals, and then return to Vindicator Vedaar at Forest Song in Ashenvale.
I will not tolerate their existence on this world! The Burning Legion all but destroyed our race and we fell back to settle a new home on a new world. Then they came again. And we retreated, over and over.
There is a remnant of the Legion to the south in the regions known as Felfire Hill, Demon Fall Canyon and Demon Fall Ridge. They are what is leftover from a force that once long ago invaded this world.
Go and destroy as many as you can before it is too late!