The Forging of Quel'Serrar

Take the unfired ancient blade to the lair of Onyxia and engage her in battle.

Some trickery must be utilized for the heating of the blade. While in combat, she will surely try to incinerate you with her fiery breath. You must plant the unfired ancient blade nearby and hope that the fiery breath heats the blade. If you live beyond this, pick up the now heated ancient blade and finish off the dragon.

Plunge the heated ancient blade into her smoldering corpse to fashion the treated ancient blade.