Diancie appears as a princess in Diancie and the Cocoon of Destruction where she was unable to produce a large diamond in order to save diamond ore. She needed more power to create such big diamonds and at the moment, her diamonds lasted for five minutes only. On elders' advice, she went on search of Xerneas to get some energy to form a diamond. However three thieves chased her in order to get diamonds.
When Ash and co. met her, they decided to help her to get to Xerneas. But they were interrupted many times by thieves. However they managed to reach to a forest where cocoon of destruction, Yveltal resided. Due to continuous disturbance, Yveltal woke up and went on rampage started to destroy everything by making them stoned. Xerneas then aids Diancie to get to mega form and to create large diamond and thus ends Yveltal's anger.