Xuan Yi intentionally became a devil to avoid becoming the Chief God and sacrificing himself for the prophesied catastrophe that Shang Gu shouldered. He attacks the God Realm while in its weakened state. Bai Jue fulfills his promise to Shang Gu to protect all the realms. 60,000 years have passed, and many things have changed. (Source: frankly at MyDramaList)