The official Japanese Pokémon website has updated with an announcements on the newly revealed Pokémon Diancie, and its role in the new Pokémon movie, including a plot summary. The articles reads as follows:

Revealing the mythical Pokémon Diancie!!

Diancie, the Pokémon that prides itself on being the world's most beautiful!
A new mythical Pokémon has just been discovered in the Nintendo 3DS games "Pocket Monsters X" and "Pocket Monsters Y"; Diancie. This is a Pokémon that can not be obtained during normal play! It's an extremely rare Pokémon which shining pink body is said to be the most beautiful in the world, and which has captivated rich people all over. In battle, it is capable of creating large amounts of diamonds between its hands to protect itself from enemy attacks!


The Mythical Pokémon Diancie
Species: Jewel Pokémon
Type: Rock/Fairy
Ability: Clear Body
Height: 0.7m
Weight: 8.8kg

Diancie will also play a major role in "Pokémon the Movie XY: The Cocoon of Destruction and Diancie"!
The Pokémon movie "Pokémon the Movie XY: The Cocoon of Destruction and Diancie" will premiere Saturday July 19th!The newly discovered mythical Pokémon "Diancie" will play a major part, having the lead role in this movie!

Diancie is the princess of the "Diamond Ore Country", where this movie is set. Diamond Ore Country is a beautiful land located deep underground, where the "Jewel Pokémon" Melecie live in peace and prosperity. However, this country is about to lose its brightness, and eventually fall into ruin......

In the movie, Diancie sets out on a journey to save her country and encountering Satoshi and Pikachu; a journey in search of the "Life Pokémon" Xerneas, seeking its sacred power. However, a cocoon lies in wait for them... a cocoon where the "Destruction Pokémon" Yveltal, which is said to have once eradicated all life in the Kalos region, sleeps....

Information on the 2014 Pokémon movies!
They will premiere nationwide Saturday July 19th!
The Pokémon movies are entering a new dimension!
The 2014 Pokémon movie will be a double feature: "Pokémon the Movie XY: The Cocoon of Destruction and Diancie" and its co-feature "Pikachu, What Kind of Keys are These?"!
In the epic adventure feature film "The Cocoon of Destruction and Diancie", Satoshi and friends get involved with the mythical Pokémon "Diancie" and the two great Pokémon of the Kalos region. And in the short "Pikachu, What Kind of Keys are These?", cute Pokémon set off on a huge adventure through a mysterious world.
Stay tuned for lots and lots of further information on these movies in the future!

For further information, visit the official Pokémon movie website

Announcing the full title of the 2014 Pokémon movie!

"Pokémon the Movie XY: The Cocoon of Destruction and Diancie"
We're hereby announcing the full title of the Pokémon movie to be released Saturday July 19th, 2014! It will be called "The Cocoon of Destruction and Diancie".
The movie will star the newly revealed mythical Pokémon "Diancie" in the lead role!

Story Outline
Diancie is the princess of the "Diamond Ore Country", where this movie is set. Diamond Ore Country is a beautiful land located deep underground, where the "Jewel Pokémon" Melecie live in peace and prosperity. In this country, a giant diamond known as the "Sacred Diamond" can be found...

For ages, the Sacred Diamond has played the role of an important energy resource, which keeps the ore country going. The only being with the power to create this type of diamond is princess Diancie herself.

However, Diancie currently does not possess this power... and time is starting to run out. The Sacred Diamond's lifespan is about to run out. When that happens, the beautiful ore country is going to perish!

Having encountered Satoshi and Pikachu, Diancie sets out on a journey to save her country; a journey in search of the "Life Pokémon" Xerneas, seeking its sacred power. However, a cocoon lies in wait for them... a cocoon where the "Destruction Pokémon" Yveltal, which is said to have once eradicated all life in the Kalos region, sleeps....

We will be presenting more information on the movie later, so stay tuned!

A new series of Pokémon Movies will be coming your way in 2014!
The title of the Pokémon movies to be released in 2014 have been announced!
Its name is "Pokémon the Movie XY: The Cocoon of Destruction".
Also, it will have a co-feature titled "Pikachu, What Kind of Keys are These?"
They will premiere in theaters all over the country starting Saturday July 19th 2014!

Check out the trailer right away!

Sources: http://www.pokemon.co.jp/ex/diancie/?p003=msign and http://www.pokemon.co.jp/anime/movie/?a001=news