You are Arceus, a mythical pokemon that is said to be the god of everything. In fact, Arceus is just the god of the earth, explaining his limited power and his weakness. In this story, you're not the true arceus, that rest in space, you're one of his few copies, created to troll the trainers. If you were THE arceus, the story would be too short and easy for you (the only difficulty would be to beat all the legendaries, including the final boss, Ethernatus.
You start as a lv1 Arceus, who just hatched from his egg. As an Arceus, you have a great potential, including a bersek mode (you use thousand arms to fight), the potential to create life to get allies and servants and a super form, unlocked with the 18 Arceus plates, spread around the world (to create life and use bersek mod, you need energy, so food). Do you still want to be THE arceus ?
