Nightmare Moon's return heralded a new age of darkness for Equestria. Celestia did not stand a chance, but contrary to most pony's expectations, a cold and thankless night was not their fate.
Nightmare Moon kept warmth, heat, and life, moving through the land, probably due to some fell magic. In this new landscape, ponies need to find new lives for themselves, and Chatelaine is one such pony.
Thrown from the castle, she becomes a delivery pony to make ends meet. This delivery goes out to a village in the middle of nowhere, where she finds darkness has taken on a new life, and the village is more dangerous than she imagined.