This story is a sequel to Crystal Book

While the six major dark spirits escape their prison one other lesser spirit named Reaper escapes unnoticed. After his siblings were sealed away in the "Crystal Book" Tales he began his rise to power by heading to the badlands to build an army. Celestia and Luna take it upon themselves to hunt down and seal Reaper away as Twilight is unable to use her power and the Elements of Harmony to take on this task. Celestia and Luna meet new friends, from Equestria to the Badlands, as well as old allies as they track down Reaper.