Yveltal

"Yveltal, the Destruction Pokémon", is a hostile, one-of-a-kind, giant flying scion-like pokémon from the homonym franchise; introduced as the emblematic pokémon of the 3DS videogame Pokémon Y. It has the status of a legendary pokémon as much as others like Palkia, Reshiram or Kyurem do, along with high stats and no evolution. It is based on the Hræsvelgr.

Nature
Yveltal is a Pokemon that absorbs the life energy of living creatures, presumably to make room life made by it's counterpart, 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 life energy every proximate life form and use it as energy; 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 which is a habit that this pokémon could have every life span.

Biology
Yveltal is an giant red-and-black avian, most of his physiology is dark-colored. It has five appendages aligned in all over its body in which the odd three ones are prominent and each end in five claws, the ones at the extreme ends are used as wings to keep this pokémon airborne and the middle one reminisce a tail. The other two even appendages resemble talons and are remarkably shorter. The pokémon has also blue eyes, a greyish ruff covering the lower diameter of its long neck and black horns that extend all the way from its snout, the trident-like patterns adorning the prominet appendages also protrude from its beak-like snout. As told by Professor Sycamore, this pokémon 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
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.

Argus and Millis Steel, Marilyn Flame and Ninja Riot were fighting over who should catch Diancie. They eventually took their fighting to an underground pool in which 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 and 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.

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, and destruction, while Xerneas represents life, peace, and creation.
 * 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.
 * 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.