Enter Ragefire Chasm and slay Taragaman the Hungerer, then bring his heart back to Neeru Fireblade in Orgrimmar.
The primary task set upon me by our great Warchief is to root out the creatures responsible for infesting our lord's great city with demonic influence. The Burning Blade is one threat, but there are others; the Searing Blade for instance, who make their home in Ragefire Chasm, secretly attempting to subvert innocent members of the Horde.
If they are to be stopped, then their leader must be slain--a Felguard named Taragaman the Hungerer.
Kill him, and his heart will appease Thrall, of this I'm sure.