Diathorus the Seeker resides somewhere in the barrow den atop Demon Fall Ridge.
Return to me with their weapons as proof of your deeds.
SPOILER ALERT : HOW TO DO THIS QUEST Edit
DON'T READ THIS UNLESS YOU WANT VERY SPECIFIC INFO ON THIS QUEST'S GOAL'S LOCATIONS!
Assuming you know how to find Felfire Hill from an earlier quest, find Demon Fall Canyon by following the road thru Felfire Hill. After passing a small dead end "path" to the left, you come under an arch of stone. There is an upward path to the left and a path basically straight ahead, tending right. Follow the path to the left. When you get to the top, go left. Gorgannon is at the end of this path. Kill him and get his Sword. Return the way you came, back to the arch. Follow the straight ahead path. Hug the right wall and look for a path on the right; it's start is marked with a light and it is lined lights: this is Demon Fall Ridge. Follow it up to the top into an open area. The barrow den is basically straight ahead. Enter the den and follow the only path. You come to an area with a ramp going down on the left and two bridges to the ramp's right. Diathorus is in the chamber across the left bridge.
Those dreadlords must die soon, <name>!
Never again will the Burning Legion make slaves of the orcs, or any other race of the Horde!
Bravely fought, <class>! You have the true heart of the Horde beating inside of you! You give us much honor!