Yun Xiaoyao did this because Yun Qing'er had told him that this medicinal liquid could elevate his physical body to the Divine Body Realm.
At first, Yun Xiaoyao did not believe it. After all, advancing to the Golden Body Realm had been painful enough, let alone reaching the Divine Body Realm.
Later, however, Yun Qing'er wagered her personal Emperor Weapon, which finally convinced Yun Xiaoyao to give it a try.
Chapter 454