What's Wrong with Becoming a Demon?

Saints and demons are born according to the will of heaven and earth. After birth, immortals and demons become one body, becoming immortals during the day and demons at night. But when he reaches adulthood, he can abandon one of them and become an immortal or a demon forever! When you become a demon, you can dominate the world and rule thousands of people! Becoming an immortal can control the wind and rain of a world and protect it. He can be a gift left by the world to every era, and it can also be a restart in silence.