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.
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(Source: serebii.net)

Pokémon the Movie: Diancie and the Cocoon of Destruction