The Celestial Capital Soundless Thunder Sword transforms the Supreme Yang Fierce Thunder into Ultimate Yin and Soft. This Yin Yang Transformation is extremely intricate, and was created by the flash of inspiration from the Celestial Fool, resulting in such a profound mysterious technique.
However, ultimately, Celestial Fool lacked depth and sufficient wisdom. He created the Celestial Capital Soundless Thunder Sword, but couldn't truly perfect it even after thousands of years.
Moreover, Celestial Fool did not realize that the most important issue was achieving the Yin Yang Transformation of external power.
Gao Xuan obtained the Celestial Capital Soundless Thunder Sword from Celestial Fool and immediately grasped the key to this Secret Technique.
Although Gao Xuan did not have the Formless Sword to depend on, his Divine Soul contained the Yin Yang Five Elements Rune Seed deep within.
