I've been pondering over this piece from the books since I saw today's episode...
The Legend of Lightbringer
Darkness lay over the world and a hero, Azor Ahai, was chosen to fight against it. To fight the darkness, Azor Ahai needed to forge a hero's sword. He labored for thirty days and thirty nights until it was done. However, when he went to temper it in water, the sword broke. He was not one to give up easily, so he started over. The second time he took fifty days and fifty nights to make the sword, even better than the first. To temper it this time, he captured a lion and drove the sword into its heart, but once more the steel shattered. The third time, with a heavy heart, for he knew before hand what he must do to finish the blade, he worked for a hundred days and nights until it was finished. This time, he called for his wife, Nissa Nissa, and asked her to bare her breast. He drove his sword into her breast, her soul combining with the steel of the sword, creating Lightbringer, while her cry of anguish and ecstasy left a crack across the face of the moon.[1]
According to the Jade Compendium, since that day the sword became as warm as Nissa Nissa was and burned fiery hot during battles. Lightbringer was able to boil the blood of a monster when Azor Ahai thrust the sword through the belly of the beast. After steam poured from the beast's mouth and its eyes melted, its body burst into flame.