I think its Nature and Decay. I think it would be amazing if they did a Fall of Lucifer type deal with Yveltal.
Yveltal was once a beautiful Pokemon whom was Arceus' Great Herald who would fly down to Earth and pass Judgement on behalf of Him.
Yveltal had amazong power but also a curse. If he ever touched the Land he would be trapped ilon that plain forever. Never able to return to the land of thr Gids in the clouds abovr Yrradasil.
He lived upon the top of Yrradrasil, bathing on Arceus light and power but eventually he grew tired of merely being a Herald. He wanted Godhood and challenged Arceus for more power but was utterly defeated.
Arceus then forced him to hit the Earth and robbed him of his rank, taking away his Holy appearance, transforming him into the vulture like appearance we know. Arceus then gave him a new task. He would be the Cleaner. He once led but he shall forever be behind. Retaking the deceased from the world so they could be reborn.
Yveltal was still quite strong even after being stripped of his Holy power and swore that he would retake his place in Heaven. Arceus than created Xernes to protect the gates of Heaven, the base of Yrradrasil, from being opened or destroyed by anything or anyone, even Yveltal himself.
Being so close to the Tree granted Xernes great power and his abilities became similar to the tree. Creating life.
As a result, Yveltal tries to lead Xernes away from Yrradrasil by leading the world into chaos and decay.
