Source from: http://www.youtube.com/watch?v=ViW5WPZZ7cQ
Info: Nickelodeon will treat Avatar: The Last Airbender fans to a unique experience this September with a brand-new, multiplayer global online game. In the game, Avatar: Legends of the Arena, which will serve as a prequel story to the series, players can create their own Avatar character and test their skills against players from around the globe in several 3D online arenas. Each game level will introduce and unlock new content, challenges, custom character outfits and more on an ongoing basis.
the game will be here
http://www.avatarlegends.com/(the website doesn't work yet)
and the beta version of the game will be unveiled at Comic-Con International in San Diego on July 25