You may a while back that a hacker sucessfully hacked into Pokemon X and Y to reveal supposedly Legendary Pokemon hidden in the game's code, though one couldn't be sure if these Pokemon were real or fake. Now, one of those Pokemon, Diancie, has been officially announced to be a Rock/Fairy type Legendary. Diancie is said to result from a "sudden transformation" of Carbine, and the Pokemon will appear to have a significant role in the new Pokemon movie.
Check out a trailer, and some screenshots of Diancie below:
You can find out a bit more about Diancie on the official Pokemon website. The method to obtain a Diancie has yet to be announced.