You must get Onyxia to breathe fire on the Unfired Ancient Blade. Once this is done, pick up the now Heated Ancient Blade. Be warned, a Heated Ancient Blade will not remain heated forever - time is of the essence.

The last step before returning to me is to slay the beast and drive the Heated Ancient Blade into her corpse.

Do this and Quel'Serrar will be yours.

Chain

Foror's Compendium → The Forging of Quel'Serrar → The Forging of Quel'Serrar