Introduced in Pokémon X/Y is a new form of evolution called Mega Evolution. Unlike normal evolution the effects are not permanent and last until the battle is over where it then reverts back to its normal form. How does it work? With an item called a Mega Ring (XY) or Mega Bracelet (ΩRαS) and the appropriate Mega Stone as its held item a Pokémon can Mega Evolve. Using Mega Evolution can change a Pokémon's appearance, stats, ability, and even their type. Mega Evolution is not without its limits as it can only be used once per battle and is only available to a select few Pokémon.
Lucky for us Diancie is one of those few who can use Mega Evolution! It is interesting that currently Diancie is the only Mythical Pokémon who can use Mega Evolution. I'm not sure why that is but I'm not gonna complain, because Mega Diancie is a blast! I'm getting a little ahead of myself though as first you need to get yourself a Diancie. If you are lucky and manage to get one be sure to leave the Pokémon Center and come back to trigger an event so you can get the Diancite, the Mega Stone required in order to Mega Evolve Diancie. I don't quite remember what it all entails, but from what I remember some guy begs you to give him Diancie for his kid until said kid comes in and says he doesn't want it and then gives you the stone before they leave. It has been a while so I may have something wrong.
Okay, so now you have a Diancie and you can use Mega Evolution. What's all different? Well, aside from an incredibly gorgeous Mega Evolution form a lot of things have changed. For one, Diancie's Ability is now Magic Bounce. I explain what it does over in Strategy, but for a quick overview Magic Bounce basically reflects status ailments and stat reduction moves back at the opponent. While Diancie's type remains Rock/Fairy, its stats change a lot. It's actually kinda interesting I think. Mega Diancie gains a huge boost to its Attack, Special Attack, and Speed, but in return it loses points in its Defense and Special Defense. I guess Mega Diancie would be ridiculously overpowered otherwise. That's okay though because it is still amazing.