Xerneas is a blue and black, quadrupedal, cervine Pokémon with an eight-horned set of antlers extending out from its head, four on each side. Xerneas's coloration changes slightly depending on what mode it is in: Active Mode or Neutral Mode. In Active Mode, each horn is pale gold and adorned with a colorful, three-part glowing protrusion twining through the horn (orange, red, purple, and teal-blue), one side mirroring the other. The colors gradually turn lighter to darker when moving from base to tip. In Neutral Mode, the horns are pale blue and the protrusions the same blue as its upper body. Xerneas's head is blue with two horns on either side of its head, and the root of its horns mark its face, just below the eye and over the nose. Its eyes are the same color as its fur, and its pupils are X-shaped. The blue portion of its body ends at the chest area where the "fur" juts outward to the side, similar to the blue horns on its face. On its breast is an inverted V-shape that is also pale gold in Active Mode and pale blue in Neutral Mode. The inside of the V-shape is black and extends into its lower body.
The rest of its body is black with a blue stripe running along its spine; when it is in Active Mode, the shine from its horns decorate its back in small, multicolored spots. It has a slim, fluffy black tail where the blue stripe ends. Its legs are fashioned like blades with forward-pointing "hooks" at the tips, creating a cross shape, with the main "edges" trimmed with the same color as the horns (pale gold in Active Mode, pale blue in Neutral Mode); the trim on its forelegs further map a crude sword-like pattern: a long blade, a cross-guard, and a hilt. Xerneas has the power to share eternal life; this occurs as the horns on its head shine in seven colors. When Xerneas is at the end of its life, it releases all of its life energy into all the living things in around it and transforms into a tree to sleep for 1,000 years. Xerneas also has the power to create forests, making Xerneas a benevolent force in nature. Xerneas is the only known Pokémon capable of learning the move Geomancy.
- Xerneas will soon meet in Team Robot in Pokemon: Diancie & The Cocoon of Destruction,
Hundreds of years prior to the movie's events, Yveltal almost destroyed all life in Allearth Forest. Xerneas saved Dace and many other Pokémon from the destruction, and later reversed all the damage Yveltal did.
Diancie, unable to create diamonds without them disappearing after a few seconds, sought after Xerneas in hopes that it would give her the power to create diamonds that would not disappear, as if the Diamond Domain's Heart Diamond disappeared, the Domain could collapse, and Diancie was not able to create a new one. When Diancie met Xerneas, it revealed it was aware of Diancie's reason for seeking it, and told her that the power was within her. It then granted Diancie what she was looking for: its Fairy Aura. Soon after, it gained a telepathic connection with Diancie.
Later, after Yveltal awakened again and had petrified large areas of the forest, Team Rocket, Marilyn Flame, Ninja Riot, The Steels, Major Nixel, Verminious Snaptrap, The Villains, Bird Brain, Etemon, Zach Varmitech, Donita Donata, Gaston Gourmand, Bat, Owl, Zippy, Zackbots, Mal, Cockroaches, Mutated Animals, Team Robot's Allies, 3 DigiDestined and Their Digimon, the Carbink servants (save for Dace), Pikachu and Mane 6, who tried to battle Yveltal) with Oblivion Wing, Xerneas eventually protected Ash, Emerl and their friends from Yveltal's Oblivion Wing by using Aurora Beam to counter it. It then used Fairy Aura to calm Yveltal's rage. After Yveltal left the forest, Xerneas used Geomancy to revive everyone and everything Yveltal had petrified. While using Geomancy, Xerneas began changing into its tree form. When asked if this meant that it was dying, it responded that it was simply beginning a new life.
Xerneas was seen in the credits remaining in the form of a tree, watching over the Allearth Forest.