Destroy a Fel Reaver in Hellfire Peninsula and plunge the Unfired Key Mold into its remains. Bring the Charred Key Mold to Rohok in Thrallmar.
It's no good, <name>! No matter how many coals I put to the fire, it's just not enough heat.
There's only one thing I can think of that is hot enough to temper this key and it is the insides of the Legion's fel reavers.
I've only ever seen one downed once. I remember seeing its insides fuming, shooting fire and steam as it fell. Take this mold and plunge it into a fallen fel reaver. I recommend you get yourself some friends if you plan to take one down.