Xerneas appeared in Diancie and the Cocoon of Destruction as a being of nature where Diancie was searching for it to get Fairy Aura in order to create a new Heart Diamond. Xerneas moved from place to place which made it hard for Diancie and her Carbink to locate it. After Diancie encountered Xerneas it granted her Fairy Aura to create a Heart Diamond. However when Yveltal started to destroy the forest, Xerneas reappeared and battling it, and then Xerneas uses its Fairy Aura to calm Yveltal down halting its rampage. After that, Xerneas restored the Allearth Forest and all the people who were hit by Oblivion Wing back to life. After restoring the forest it began transforming into a tree to protect it.
Mary O'Brady (English)