The first and eldest son of the Heavenly Emperor of the Celestial Realm and a Dragon Fish Princess of Dongting Lake, Run Yu is an immortal with water essence. He is the god in charge of the night and only has the nightmare beasts as company. He has to keep a relatively low profile and a reclusive life in the Celestial Realm as the Heavenly Empress is constantly wary and suspicious of him because she fears that he would one day usurp her son, Xu Feng's, place as heir to the throne of the Realm.