The story is set in Diamond Ore Country, of which Diancie is the princess.This is a country located deep underground, where the Pokémon Carbink live together in peace. The energy source of this country is a giant diamond called the Sacred Diamond, which can only be created by princess Diancie. However, the current princess doesn't yet possess the power to create such a diamond, and the current Sacred Diamond's power is soon going to run out, which would result in the end of the country. Diancie meets up with Ash and Pikachu, and set off together with them to search for the legendary Pokemon Xerneas, which possesses sacred power, but on the way, they find the cocoon where the Destruction Pokémon, Yveltal, who was said to have once destroyed life in Kalos, lies in wait.
 
(Source: serebii.net)

Satoshi and Pikachu have arrived in Lumiose City of the illustrious Kalos region to capture more Pokémon and continue their journey toward becoming the very best. Meanwhile, a genius inventor named Citron and his little sister Eureka wander the city when they run into Satoshi who quickly challenges them to a battle. However, they are soon caught up in a dangerous incident when Team Rocket, following Satoshi into Kalos, cause a Gaburias to rampage through the city.
 
Far away in the quiet Vaniville Town, a young girl named Serena slogs through daily Sihorn riding practice at the behest of her mother, a professional Sihorn racer. After practice, she sees the events unfolding in Miare City on television where she recognizes a boy from her childhood. Having left a significant impact on her life, the sight of him stirs in her a desire to meet him again; and so, Serena sets off to Miare City, determined to find the boy from her past.
 
Pokemon XY follows the group as they travel throughout Kalos in pursuit of their ambitions—Satoshi challenging Pokémon gyms, Citron learning from Satoshi, and Serena searching for what exactly her dream is. Along the way, they meet new friends, face new rivals, and continue to thwart Team Rocket's schemes, all the while discovering a little about the mysteries of mega evolution.
 
(Source: MAL Rewrite)

Prequel to the 17th Pokémon movie, Pokemon XY: Cocoon of Destruction and Diancie.

A short which ran along side the Pokemon XY: Hakai no Mayu to Diancie movie.

In the underground Diamond Domain, where many Carbink live, the Mythical Pokémon Diancie serves as ruler. The Heart Diamond that sustains the land is beginning to fall apart, and Diancie is not yet strong enough to create a new one. While seeking help from the Legendary Pokémon Xerneas, Diancie encounters a group of thieves who want to take control of its diamond-producing power and who awaken the Legendary Pokémon Yveltal from its cocoon in the process! Can Ash and his friends help Diancie discover its true power, stop Yveltal’s rampage, and save the Diamond Domain?! 
(Source: VIZ Media)

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