In Pokemon Black for the Nintendo DS, you have a limited time to catch Victini since it’s tied to a WiFi event. This event will be over on December 31st, 2011 for the US version so get it while you can.
To catch Victini, you need to have a WiFi connection setup on your DS, and from the main menu connect to receive the Mystery Gift, which is now a level 50 Victini.
This version of Victini comes with a new move called V-Create, which lowers defense, speed, and special defense.
This Victini also comes with Fuson Flare and Fusion Bolt, which effect each others damage. Basically, if you can do both moves in one turn, the second will do double damage.
The fourth move that Victini comes with is Searing Shot, which is just a fire based attack that has a 30% chance to leave the enemy burned.