Tataka said that since your skills have improved your weapon should also be improved. You obtained the Vorpal Essence, Special Shulack Refiner, and Blessed Aurium Ingot, and went to the Jotun Studio. You forged them together with the Noble Siel's Supreme Greatsword to make a crystal called the Essence of Siel. When you brought the Essence of Siel to Tataka, he made you a Greatsword of Siel's Power.