Crystal Hearts is a side story of Kkat's Fallout: Equestria, and written by ChurchNW. It is currently at one chapter, and over 10,000 words.
The Fimfiction synopsis reads: "After her apotheosis, the Holy Mother provided the Crystal Wasteland with The Book of Cadance. The book’s purpose was to guide the crystal ponies into an era of love and harmony, but left no mention of the other races. While those heathens have it rough, life isn’t easy for the chosen ponies either. As a young crystal pony embarks on a pilgrimage, the trials of the Crystal Wasteland will shed light on two questions. What does it mean to be a family; what is worth having faith in?"
The story is set in the Crystal Wasteland. Here Cadance is worshiped as a Goddess, with crystal ponies being seen as the chosen ponies. Little else is known about the Crystal Wasteland at this time.