What novel is China's magic weapon in Aauto Quicker's video?

Wei Zi soft sword is a magic weapon in Jin Yong's martial arts novel The Condor Heroes. The holder is a young Demonbane who seeks failure alone. The body of the sword is about three feet long, and the purple light flashes, which is extremely soft and emits a purple gas like smoke and fog. The broken gold is as sharp as mud, and there are two gold thread seal characters on the hilt, which are "Wei Zi".