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The Diamond Domain (Japanese: ダイヤモンド鉱国 Diamond Ore Country) is an anime-exclusive location in Kalos. It is one of the settings of Diancie and the Cocoon of Destruction. It neighbours the Allearth Forest.
It is a beautiful, underground domain where many Carbink live in peace and Diancie serves as its ruler. Among these many Carbink, four are named, and have a close association with Diancie. The Heart Diamond is a large diamond created by Diancie, which is used to sustain the land and provide the Diamond Domain with energy and tranquility.
One day, the Heart Diamond was on the verge of losing its energy due to its old age. According to Dace, an elderly and wise Carbink, should the power of the Heart Diamond would die out, the Diamond Domain would be finished. As such, it gave Diancie the task of creating a new Heart Diamond, as only she had the ability do so. However, she failed to complete this task, lacking the power needed to create a new one. Due to this, Dace now tasked Diancie with finding Xerneas, whose Fairy Aura would be give Diancie the power she needed to be able to create a Heart Diamond. Diancie and her Carbink servants then left the Diamond Domain to fulfill this task.
Later, Diancie and the Carbink servants returned to the Diamond Domain together with Ash and his friends, partially to seek shelter from thieves hunting down Diancie. During the time Diancie had been away, the Diamond Domain had begun fallen in despair, as the Heart Diamond was now completely dead. After this, Ash, his friends, Diancie and the Carbink servants left the Diamond Domain again to journey to the nearby Allearth Forest, this time accompanied by Dace, as Diancie sensed Xerneas's aura coming from here.
After Xerneas had driven off Yveltal inside Allearth Forest and returned the balance of nature, the group, Diancie and the Carbink servants returned to the Diamond Domain once more. Diancie used her newfound power from Mega Evolution to create a new Heart Diamond that would be able to sustain the Diamond Domain with new energy and tranquility for years to come.