From Bulbanews, your community Pokémon newspaper.
Legendary Pokémon available from Dec. 16 to Jan. 10
Tornadus and Thundurus in the anime
Level 70 Thundurus and Tornadus will be distributed via Nintendo Wi-Fi Connection from Dec. 16, 2011 to Jan. 10, 2011 to Japanese Black and White games to commemorate the upcoming two-part anime episode 「トルネロスVSボルトロスVSランドロス！」 Tornelos VS Voltolos VS Landlos, the first part of which will air on Dec. 15.
The event allows players to obtain the Pokémon not obtainable in their Black and White games, with Thundurus available to those with Pokémon Black and Tornadus to those with Pokémon White. This allows players to obtain Landorus without trading.