In order to Mega Evolve Diancie into Mega Diancie, you'll have to start by obtaining Diancie for your Pokémon X or Pokémon Y game. Then, bring the Diancie you received into Pokémon Omega Ruby or Pokémon Alpha Sapphire! Stay tuned for information on how to get Diancie.
The impurities upon its body’s surface have fallen away, and it sparkles so brilliantly when reflecting light that you cannot look directly at it. The diamond on its head measures over 2,000 carats.
Diancie can do even more when it Mega Evolves, changing its Ability and its stats. With the Magic Bounce Ability, any status conditions or stat-lowering effects can be bounced back at the one trying to inflict them. Also when Diancie Mega Evolves, its Speed increases greatly. Mega Diancie's Defense and Sp. Def both go down, but its Attack and Sp. Atk gain big boosts, making its Mega Evolution perfect for getting off fast attacks.