Diancie is a new rock/fairy type Pokemon for Pokemon X and Y.
Every generation of Pokemon features some ultra-secret 'mons that are usually only available via a special distribution event. Now, Nintendo UK has revealed that the first of these such Pokemon will be Diancie, with a Rock and Fairy typing. It didn't announce specifically how and when Diancie will be made available, but said details are coming soon. It will most likely be via a wi-fi download, much like the special Blazikenite Torchic event accompanying the game's release.
"Diancie can create diamonds out of thin air by compressing the carbon in the atmosphere with its hands. It uses these jewels in battle to attack enemies and protect itself."
You may recall that Celebi was made available to early adopters of the Pokemon Bank. However, Celebi is actually from an older generation of Pokemon games. Diancie will be the first, completely new legendary Pokemon to be distributed since the game's launch.
However, those of you who are savvy to the Pokemon scene, may actually remember Diancie. Shortly after the game's release, a hacker managed to discover three, secret, unobtainable Pokemon, and sure enough, there's Diancie right at the top. This means that the other two Pokemon he found are also most likely legit, and should be officially revealed by Nintendo at some point in the future.