After last months official acknowledgement of Diancie by Game Freak, hackers of Pokemon X and Y have unlocked the event in which the player encounters the legendary Pokemon.
The Pokemon appears to be received through Mystery Gift, which means that Nintendo will eventually release an online event in order to get Diancie.
The Rock/Fairy type comes at level 15, knowing the moves Harden, Rock Throw, Sharpen and Smack Down. It's ability is Clear Body, which prevents the opponent from lowering its stats.
Once received, Diancie triggers an event involving two people that are trying to get the legendary Pokemon for one of their higher ups. You can watch the whole thing in the video above, though it is a fairly bland narrative.
Since this is hacked, there is still no news on when Diancie will be released for all owners of Pokemon X and Y. It will more than likely coincide with the July 19 of the newest Pokemon movie, which stars Diancie.