Upper Realm, Heavenly Domain.
Several Gods and Goddesses could perceive the power differences between the King of Demons and the Immortal Emperor due to their days-long battle. Even though it is obvious that the Emperor of the Heavenly Kingdom is stronger than the Demon King, the latter possessed a tougher body that allowed him to defend himself from the former.
Their conflict was never life-or-death, but rather a demonstration to the public that the two were on opposing sides. After all, on his journey into the Ice Jade Palace, the King of the Demons had slaughtered several Palace Guards.
