Being princess for 1000 years means you've received more than a few threats, but somehow Celestia has maintained peace across all of Equestria. It isn't until she is attacked by a mysterious unicorn and her cutie mark is stolen, that all this changes. Celestia is transformed into a husk of her former self. Not alive but not dead either. Twilight soon learns of the Legend of the Harbingers and is forced on a quest to gather the Elements of Harmony to vanquish this new evil. If she fails, all her friends, all of Equestria will be erased from existence.
So if you feel in the mood for some fan-fiction, be sure to check it out ^.^