Everything Pokémon: Pokémon Facts Book 2

Pokemon Myths #9

Aura Trio

Xerneas and Yveltal are great forces, being capable of giving and stealing life energy respectively. Xerneas shares life by glowing its horns in seven colors, while Yveltal's feathers near the gray pattern on its chest glow when it is about to steal life energy. When they are at the end of their lifespan, Xerneas releases all of its life energy into all the living things in around it, while Yveltal expands its wings and steals all of the life energy of every living thing around it. Afterwards, they transform into a tree and a cocoon respectively, to sleep for 1,000 years. Zygarde Cells are always monitoring the balance between Xerneas and Yveltal, and whenever it is disrupted or destroyed, Zygarde comes to unleash its wrath. These three Pokémon, according to Professor Sycamore, are "incarnations of nature that take care of the land".

A Monsieur on Anistar City recounts the tale of the Mascot Legendary Pokémon of Generation VI with the following tale:

Xerneas

"It happened around 800 years ago. The Pokémon known as Xerneas used its shining horns to illuminate the lands of Kalos. At that very moment, people and Pokémon throughout the land felt great energy and vitality surge through their bodies. At the same time, a vast forest sprang up, with Xerneas at its center. Legend has it that, when it nears the end of its thousand-year life span, Xerneas releases all of its remaining energy, sharing it with all living things nearby. There's another story I know that talks of the appearance of just such a fortuitous Pokémon. Back 3,000 years ago, the Kalos region was engulfed in a terrible war. So the tale goes, a Pokémon came to the rescue of all the wounded Pokémon that had been caught up in the conflict. Some people suspect the Pokémon that appeared was Xerneas, but there's no way to know. Maybe they just wished a Legendary Pokémon had come to the rescue to brighten a tale of woe. Supposedly, after Xerneas had released its remaining energy, it transformed into a dried-up tree. It remains in hiding deep within the forest. According to old tales, as I say. Stories and tales."

Yveltal

"It happened around 800 years ago. The Pokémon known as Yveltal spread its ominous wings, engulfing the lands of Kalos in darkness. At that moment, people and Pokémon throughout the land began to fall one by one. Yveltal let out a piercing cry and took to the sky, vanishing to an unknown location. Stories say that when it nears the end of its thousand-year life span, Yveltal absorbs the life force of the living things around it in order to charge its own energy. Another tale talks of the appearance of just such a terrifying Pokémon. Back 3,000 years ago, the Kalos region was engulfed in a terrible war. According to the stories, a Pokémon appeared and stole the life force of countless living beings. Some people suspect the Pokémon that appeared was Yveltal, but there's no way to know. Maybe they're just using a Legendary Pokémon as a metaphor for war and pestilence. In the tales, after Yveltal finished storing the energy it had absorbed, it transformed into a kind of cocoon, remaining in hiding deep in the mountains. According to old tales, as I say. Tales and stories."