Kill Gorgannon and Diathorus the Seeker, and then return to Velusha at Splintertree Post in Ashenvale.
[player], you will see to it that this attack by the Burning Legion never comes to pass! Go and slay these dreadlords! Without them the rest of the demons are essentially mindless.
You will find Gorgannon in Demon Fall Canyon south beyond Felfire Hill, near Mannoroth's floating, broken spear.
Diathorus the Seeker resides somewhere in the barrow den atop Demon Fall Ridge. To find the ridge, look for a ramp up from Demon Fall Canyon near the monument to Grom Hellscream.