The Moon's Embrace by GoG ToXiC
Link - http://www.fimfiction.net/story/84612/1/the-moons-embrace/the-moons-embrace

"Luna spent a thousand years locked away on her moon, isolated from all traces of society. Now, despite her return from exile, and her new friends surrounding her, the memories of suffering still linger. Can a visit with a particular unicorn, one that means more to her than she might realize, banish a millennia's worth of pain? Or will the moon, once the pride of her existence, remain out of reach forevermore?"