Collect 10 Fel Orc Blood Vials and return them to Centurion Caza'rez at Thrallmar in Hellfire Peninsula.
That letter is disturbing, and not just because it comes from the Betrayer. We must know how they are creating these new fel orcs if we're to defeat them! It's bad enough that we must fight the Burning Legion on our eastern flank, but now a new army of fel orcs to the west!
You must infiltrate this Blood Furnace inside Hellfire Citadel. Bring back samples of blood from the fel orcs that you find in there.
Surely you have not collected all of the blood so quickly?
I am impressed with your abilities, <name>. Not once, but twice now you have infiltrated Hellfire Citadel for us and accomplished what was asked of you.
I will see to it that this blood is delivered to those who can unlock its mysteries. Who knows, perhaps we can find a way to reverse the process?