Luna had been trapped away for almost 1000 years and her mind had been shattered. A broken shadow of what she once was, Celestia watched her sister with sorrow, unable to reach out for her. Despite her care for her sister, she saw signs of the return of Nightmare Moon and knew she needed to prepare for the worst.
With a painstaking attention to canon, this prequel tells the stories of Celestia's search for the Elements of Harmony and of Luna as she attempts to escape from the moon with the help of the stars.
Thanks to *Equestria-Prevails* for letting me use their imagining of Concordia and Princess Rosetta. Sorry if I deviated a bit from the description.