After Skyward Chronicles(Equestria Chronicles), as in the eighth installment of the Battle Path series.

Luna with the past of Nightmare Moon lives in Hion until an unexpected airship crash. Owner of the airship, Sweetie Belle (18 years old) is a survivor of the crash with Applebloom(19 years old). Luna agrees to help them survive in the darklands until the airship is fixed. Not long after an enemy called the emperor drags Luna into the conflict and in order to survive, Luna must use her power of Nightmare Moon.

just a random earth pony takes half credit for this story.