What is the reason for leaning on the heavenly sword?

The prototype of the eternal sword is the black iron sword, which was originally a solitary sword seeking defeat. Later, Yang Guo got the black iron sword. With this sword and the inscription that "epee has no blade, great skill doesn't work", and a big carving left by seeking defeat alone to imitate the master's practice before his death, Yang Guo realized swordsmanship. When Yang Guo realized that he had lost his soul, he wouldn't use this sword often. After Yang Guo abdicated, the sword was given to Guo Jing and Huang Rong to resist the Mongolian invasion. At the end of the Song Dynasty, when the Mongols invaded, Guo Jing and Huang Rong called on all the Wulin people to defend Xiangyang, but there was nothing they could do. Seeing that the city was about to be breached, he invited experts to melt the darksteel epee and add it to the western pure gold to make Tu Longdao and Eternal Sword.