The Crystal War is a severe conflict between the Crystal Empire and the Lunar Empire within one of the Alternate Timelines.
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The beginning of the war started with the return of the Crystal Empire, and in turn, the return of Sombra. Celestia and nightmare Moon go off to confront king sombra, but in this encounter, Celestia is killed, leaving the responsability of both day and night to Nightmare Moon. In response, she created the Lunar empire, and declared war of the Crystal Empire.
It is currently still active.