1,000 Years ago, a mysterious boy who was proficient in yin-yang and alchemy used yin-yang and alchemy to create a group of strange creatures that had human forms but could transform into illusions. This group of creatures can use the intertwined power of the formation formation and their own drinking yang to merge with humans and become a veritable blade. They often appear as ordinary humans, but they can appear as ghosts and enter human bodies. Because they can possess swords but exist in human form, humans collectively refer to them as "sword souls".