Go to Menethil Harbor in the Wetlands, and speak with Glorin Steelbrow.
Years ago, the Third Fleet of Kul Tiras sunk along the coasts of The Wetlands, an expanse of marsh north of the dwarven kingdom of Ironforge. Aboard one ship was a load of Lightforge Iron.
This metal is precious to us, for items crafted from it strike with holy truth. If you are to face Morbent Fel, then you will need such a weapon.
Go to Menethil Harbor and speak with my dwarven colleague Glorin Steelbrow. If that lost metal can be found, he'll know where to look.