Following the success of Monty Oum's animation "RWBY" alongside Rooster Teeth, a game is now in the works due to be released for Steam and currently on greenlight. If you'd like to help support it, you can vote here.

Edit: The game has been greenlit after 48 hours.

Edit edit: Game has now been released

Game Trailer

Get ready for intense combat action as you battle Grimm across familiar locations of Remnant including new areas never before seen in the show. Play as Ruby, Weiss, Blake, and Yang in this character-driven adventure that explores new storylines, new Grimm types, and a new villain!
The gameplay takes inspiration from Dynasty Warriors and Left 4 Dead offering over-the-top combat along with engaging missions and storytelling.
GAME FEATURES:

• Online Co-op Multiplayer
• 4 playable characters with unique skill trees
• Full story campaign with a NEW storyline
• MOBA-like leveling during gameplay
• Several locations from the world of Remnant
• NEW locations never before seen in the show
• NEW enemy types
• NEW villain
• Loads of unlocks, achievements, and rewards
  MORE DETAILS COMING SOON!

The game was originally created as a fan-game by Jordan Scott over a period of approximately 5 months. A demo version was released on >April 1st, 2014 (the 11th anniversary of the founding of Rooster Teeth) and quickly received widespread recognition from other fans. Upon its release, Scott stated that no further development would be made on the game as he intended to follow original concepts out of necessity.
Less than 24 hours after the public release of the game, Scott was contacted by Monty Oum and brought into discussions to develop the game for Rooster Teeth. On July 4th, 2014 during the RTX 2014 RWBY Panel, it was announced that the game was picked up by the company to be made into an official game. The game was shown off with more polished graphics, a new heads up display, and a teaser for another playable character: Yang Xiao Long.

Personally, I originally got pretty excited by the series upon viewing the teaser trailer before any content had been released. The compositions by Jeff Williams, the red and black character design of Ruby, gothic lolita style dress and scythe weapon made it exactly my forte. It made it out to be a lot more serious/dark/action+thriller than it actually ended up being. In the end, it was a lot more bubbly and bouncey than I've any particular interest in.

You may recognise Monty Oum from his other work like I did which is how I came across the series. Before going professional, he solo worked on the Dead Fantasy series which had incredibly thorough animated combat for its time for amateur work. This lead to him getting the attention of Rooster Teeth where he did animation work on the series Red vs. Blue before going on to create his own series, RWBY. Unfortunately he passed away on February 1st 2015, but I believe RT will be taking over the series and continuing its legacy.

While the game does seem to resemble a relatively generic hack and slash, at least it'll be a chance to mess around with some of the character designs in a more serious setting than the series.

Further Reading

Greenlight Page
RWBY: Grimm Eclipse wiki info
Watch the series
Monty Oum

Developer FAQ (Source)

Will the game be playable on a Mac? Will the game offer controller support on PC/Mac?

The game will have controller support (I would recommend you use a controller, actually). We're still trying to figure out the Mac thing.

How much will the game cost? Or will it be for free?

We're still trying to figure out the price but I can say with certainty that the game will not be free.

The info mentions 4 player co-op, but is single player still an option?

Yes, there will be single player. We're still trying to figure out what that single player will look like. Online co-op is our focus.

Will the online part of the game have some sort of match-making or at least public lobbies?

Yes! The game will have matchmaking!

Will the game be available on other platforms?

That's something we are interested in. Future plans are hinging on how well our Steam game does.

View attached image.