Pokémon the Movie: Diancie and the Cocoon of Destruction

In the underground Diamond Domain, home to many Carbink, 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. Ash and his friends now must help Diancie discover its true power, stop Yveltal's rampage, and save the Diamond Domain.

Synopsis from Anime News Network