The Pokémon Company has shared a little more information on Mega Diancie via the official website for Pokémon Omega Ruby and Alpha Sapphire.
As reported in the past, Mega Diancie has the Magic Bounce ability, as well as moves like Diamond Storm and Moon Blast that benefit from its stats when it Mega Evolves, seeing an increase in Attack and Sp. Atk, and reductions in Defense and Sp. Def.
Diamond Storm (above) benefits primarily from Mega Diancie’s increased Attack stat.
Meanwhile, Moon Blast, which you can see above, benefits from the increase in Sp. Atk. Additionally, it is recommended that one teach Mega Diancie the move Psychic via use of a TM to take advantage of her increased Sp. Atk as well.
Finally, The Pokémon Company revealed on the games’ website, Mega Diancie also sees an increase in Speed. We also now know that its Dazzling Gleam move hits both opposing Pokémon in a Double Battle.
Diancie will be distributed via a GameStop event in the U.S. sometime this Fall. It sounds like you’ll be able to avail it if you own Pokémon X or Y, but you’ll need to transfer it to Omega Ruby or Alpha Sapphire if you want it to Mega Evolve.