3,000 years ago, there was a war. Fighting with pokemon, killing them. Most pokemon got extinct. It was a fierce battle even legendary pokemons could not stop it. That awakens Zygarde. In its complete He made Xerneas and Yveltal to end he war. And using Xerneas and Yveltal's Powers AZ launched the ultimate weapon. It then awakens Arceus. The war destroyed both sides, leaving Xerneas to tree form and Yveltal to cocoon form. Arceus was angry beacuse Zygarde made a pokemon. He curse him. He can only revert to perfect form when needed. He will stay forever in half form while the other halg maintaining the ecosystem.