Pokémon the Movie: Diancie and the Cocoon of Destruction

In the underground Diamond Domain, home to many Carbink, the Mythical Pokémon Diancie rules over the land sustained by a Heart Diamond that is now deteriorating. Diancie lacks the strength to create a new Heart Diamond and sets out to seek help from the Legendary Pokémon Xerneas. During this journey, Diancie encounters a group of thieves aiming to exploit its ability to produce diamonds. Their actions inadvertently awaken the Legendary Pokémon Yveltal from its cocoon, causing destruction. Ash and his friends join Diancie in discovering its true power to stop Yveltal's rampage and protect the Diamond Domain.