To promote the debut of the English dub anime series Pokémon the Series: XYZ, three event Pokémon will be distributed via the Nintendo Network during May 2016.

Get a Legendary Pokémon Trio!

Celebrating the debut of Pokémon the Series: XYZ, for a short time, you can get three powerful Legendary Pokémon from the Kalos region—Xerneas, Yveltal, and Zygarde—via Nintendo Network! What's more, the Xerneas and Yveltal you'll receive are Shiny, making them incredibly rare, especially for a Legendary Pokémon! These Pokémon are all Lv. 100, and know their signature attacks as well as other powerful moves.

Be sure to look for Zygarde and more amazing Pokémon in Pokémon the Series: XYZ! Each Pokémon will be available via Nintendo Network for one week only:

May 2–8, 2016: Zygarde - Level 100 - Ability: Aura Break and know the moves Land's Wrath, Extreme Speed, Glare and Outrage.

May 11–17, 2016: Shiny Xerneas - Level 100 - Ability: Fairy Aura and know the moves: Geomancy, Moon Blast, Aromatherapy and Focus Blast.

May 20–26, 2016: Shiny Yveltal - Level 100 - Ability: Dark Aura and know the moves Oblivion Wing, Sucker Punch, Dark Pulse and Foul Play.

Don't miss your chance to get this trio of amazing Legendary Pokémon while you can!

Source: http://xy10.pokemontcgxy.com/en-us/legendary.php