A few days after the events of the royal wedding, Twilight gives news to Fluttershy to rat out any changelings she sees. This makes her anxious, so she takes a walk. On this walk, she finds something that she never thought that she would.
Nightmare Moon haunts Luna in her sleep, threatening to take control of the princess once more. There is no telling whether she will succeed and overthrow Celestia, or be doomed to another millennium on the moon.
Many years after the arc of My Little Pony: Friendship is Magic has come to a close, the passing of Twilight’s friends pushes her past the brink of despair.