A repeated prophesy from the book series. Azor Ahai/The Prince That Was Promised/The Last Hero were tales from various cultures about someone who saved the world. Many similarities in the tales.
With Azor Ahai, he was told he needed a special sword to fight the darkness. He forged one, tempered in water...it shattered. He forged another, tempered it in a lion's blood. It shattered. Finally he forged a 3rd, and tempered it through the breast of his love, Nissa Nissa...through this acdtion, Lightbringer, was forged.
Lightbringer was supposedly the sword wielded by Stannis Baratheon when Melisandre thought he was Azor Ahai reborn.