To help promote the release of the upcoming game Pokémon + Nobunaga's Ambition (ポケモン+ノブナガの野望) which will be released in Japan on March 17th, 2012, Nobunaga's Dark Rayquaza (ノブナガの黒いレックウザ) will be distributed on the Nintendo Wi-Fi Connection to players of the Japanese versions of Pokémon Black and White (ポケットモンスター ブラック・ホワイト). The Rayquaza will be distributed from March 17th, 2012 until April 16th, 2012. This Rayquaza will also be distributed at all the Japanese Pokémon Centers and at Pokémon BW Searcher (ポケモンサーチャーBW) tour locations.

rayquaza rayquaza

The details of Nobunaga's Dark Rayquaza are as follows:

Pokémon Info
pic Level: 70
OT: ノブナガ (Nobunaga)
Poké ball: Cherish Ball
Ribbon: Classic Ribbon
Ability: Air Lock
Held Item: None
Attack 1: Dragon Pulse
Attack 2: AncientPower
Attack 3: Outrage
Attack 4: Dragon Dance

Sources: http://www.pokemon.co.jp/info/2012/03/120302_g01.html, http://www.pokemon.co.jp/info/2012/02/120224_e01.html and http://game.watch.impress.co.jp/docs/news/20120302_516159.html