Yveltal

Yveltal, the Destruction Pokémon, is a hostile, one-of-a-kind, giant Pokemon from the homonym franchise; introduced as the emblematic Pokemon of the 3DS video game Pokémon Y. It has the status of a legendary Pokemon as much as others like Palkia, Reshiram or Kyurem do, along with high stats and no evolution. Its design and concept are based on the mythical Hræsvelgr.

Nature
Yveltal is a Pokémon that absorbs the life energy of living creatures, acting as a counterpart to Xerneas. Yveltal is also capable of committing one of the most harmful acts ever committed by a Pokémon that has ever been seen in the franchise: when Yveltal is nearing the end of a life cycle, it will absorb the energy of every proximate life form and use it as fuel; that way, this creature can continue living for another thousand years even if it has to revert back to a cocoon. This Pokémon is said to lie dormant in an unknown place during the game's events.

Biology
Yveltal is a giant red-and-black avian, with most of its body dominated by red and its wings and tail by blacks. It has five appendages, in which the odd three ones are prominent and end in five claws; the ones at the extreme ends are used as wings to keep this Pokémon airborne and the middle one reminiscent of a tail. The other two even ends resemble talons and are remarkably shorter. The Pokemon has blue eyes, a grayish ruff circling its neck and black horns that extend all the way from its snout, with the trident-like patterns adorning it also found there. As told by Professor Sycamore, this Pokemon is better distinguished as a giant letter "Y".

Yveltal has the power to absorb life energy. When it reaches the end of its lifespan, it expands its wings and steals all of the life energy of every living thing around it before transforming into a cocoon to sleep for 1,000 years. Yveltal is the only known Pokémon capable of learning the move Oblivion Wing.

Diancie and the Cocoon of Destruction
Yveltal is the final and true main antagonist in this film.

Hundreds of years prior to the movie's events, Yveltal almost destroyed all life in All-Earth Forest. Xerneas saved Diancie and many other Pokémon from the destruction, and later reversed all the damage Yveltal did.

Argus and Millis Steel, Marilyn Flame, and Ninja Riot were fighting over who should catch Diancie. They took their fighting to an underground pool where Yveltal slept in its cocoon form. The fighting eventually awakened Yveltal, who used Oblivion Wing to petrify large areas of the forest, as well as Team Rocket, Marilyn, Riot, and the Steels, who tried to stop it with missiles from their ship. Ash and Pikachu tried to battle Yveltal as well, but almost fell off a cliff. Diancie then Mega Evolved and created a large diamond, which she used to protect Ash, Serena, Clemont and Bonnie from another Oblivion Wing.

Eventually, Xerneas reappeared. Just the sight of it halted Yveltal's rampage, and it flew away.

Pokémon Adventures manga
Yveltal debuted in the X & Y chapter alongside Xerneas. The two faced each other in a battle, which caused chaos and destruction in Vaniville Town. The two eventually stopped fighting and returned to their dormant cocoon and tree states, which are being sought after by Team Flare to power their ultimate weapon.

Malva, a deputy of Team Flare, eventually assumed control of Yveltal, after defeating Diantha, who along with Korrina and Gurkinn had been seeking it to oppose Team Flare after believing that Team Flare were only after Xerneas.

Moves

 * Hurricane
 * Razor Wind
 * Taunt
 * Roost
 * Double Team
 * Air Slash
 * Snarl
 * Oblivion Wing
 * Disable
 * Dark Pulse
 * Foul Play
 * Phantom Force
 * Psychic
 * Dragon Rush
 * Focus Blast
 * Sucker Punch
 * Hyper Beam
 * Sky Attack

Trivia

 * Yveltal has the highest HP base stat of all Dark-type Pokémon. Yveltal is tied with Lugia, Ho-Oh, and Rayquaza for highest base stat total of all Flying types.
 * Yveltal is the first Dark-type Version mascot.
 * Yveltal has the same stat distribution as Xerneas.
 * Yveltal and Xerneas are exact opposites: Yveltal represents death, chaos, destruction and darkness, while Xerneas represents life, peace, creation, and light.
 * In a way Yveltal and Xerneas are like Yin and Yang.  This would mean Yveltal's Destructive powers are vital to maintain a balance.
 * Yveltal is the second Dark-type legendary Pokemon, with Darkrai being the first. If Yveltal will appear in any Pokemon spin-off game such as Mystery Dungeon as a villain, Yveltal will be the second Dark-type Legendary to be a villain as well as the second villain to be a Flying-type Legendary after Rayquaza.
 * In the overall story of the film, Yveltal is the primary antagonist, Argus Steel is secondary, Millis is tertiary, Marilyn Flame is quaternary, and Ninja Riot is quinary. Though Argus pretty much drives the plot of the story, Yveltal is the overall main villain because he had bigger plans and was far more dangerous than anyone else.
 * Yveltal, along with Xerneas, was designed by art designers Ken Sugimori and Yusuke Ohmura. Initial design for the two Legendary Pokémon was started by Sugimori, but he encountered 'artist's block', having a hard time coming up with concepts. For the first time, he gave the work over to another designer, Ohmura. Ohmura furthered the designs, after which Sugimori resumed to finish them.