The official movie website has updated with details on new movie-related events. Translations follow:

Movie Profile

An addition has been made to the movie's character profile list:

Picture	Biography
	Mega Diancie The mega evolved form of Diancie. Species: Jewel Pokémon Type: Rock/Fairy Ability: ??? Height: 1.1m Weght: 27.8kg

Diancie Giveaway

The Diancie giveaway event page also has an addition:


For the first time ever, a mythical Pokémon mega evolves!


Take the Diancie you receive with you to Pocket Monsters Omega Ruby/Pocket Monsters Alpha Sapphire...
And Diancie will mega evolve!

Mega Diancie
Species: Jewel Pokémon
Type: Rock/Fairy
Ability: ???
Height: 1.1m
Weght: 27.8kg

Use the link trade function, etc, to take "Diancie" from "Pocket Monsters X/Y" to "Pocket Monsters Oega Ruby" or "Pocket Monsters Alpha Sapphire", and you will be able to mega evolve "Diancie" into "Mega Diancie"!


Pursue the mystery of Mega Evolution!
Pocket Monsters Omega Ruby and Pocket Monsters Alpha Sapphire will go on sale Friday November 21st 2014

The Thieves and the 1000 Pokémon

The section on "The Thieves and the 1000 Pokémon" has received an update detailing the movie event connected to it.

If you download "The Thieves and the 1000 Pokémon", then go see the movie, "something special" will happen! Further, it can be connected to "Pocket Monsters X/Y"!!

Make sure you've added the special stage to "The Thieves and the 1000 Pokémon"
Challenge the special stage where Algus and Miriss appear!
In addition to Marilyn and Riot, the movie characters Algus and Miriss will appear as well! If you download the game, then take it with you to the theater and go see this year's Pokémon movie, you can add a special stage called the "Algus and Miriss Stage" to the game!

Rather than Marilyn and Riot, this stage is centered around Algus and Miriss! Why do Algus and Miriss take an interest in the "Diamond Ore Country"? What are they planning?
*The special stage can be downloaded no matter how far into the game you are, but it can not be played until you've beaten the regular Stage 6.

When you beat the special stage, you will receive a Master Ball in "Pocket Monsters X/Y"
When you beat the special stage, you will receive a serial code for a Master Ball present!
Jot down the serial code, switch over to Pocket Monsters X/Y, and enter the serial code there to receive a Master Ball!
*The 3DS system must be connected to the Internet for the code to work.

Before you go see the movie, make sure you've downloaded the game

In mid-June, an icon notice about the game will be added to all Nintendo 3DS systems connected to the Internet. However, this does not mean the game has been downloaded, and it can not be played in this state. Have the icon take you to the Nintendo e-shop and download the game from there.

Sources: http://www.pokemon-movie.jp/chara/, http://www.pokemon-movie.jp/theater/ and http://www.pokemon-movie.jp/topics/thief_and_1000pokemon.php