In a world of magic, it's bound that things will happen that shouldn't, or magic gets into places it was never meant to be. this is one of those times.
Princess Celestia, ruler, mentor, mother figure, and when the need arises war leader although that last one hasn't been needed for thousands of years at least. someone as long-lived as her is about to find out that sometimes the skeletons in your closet arent as dead as we think they are as an ancient enemy comes back to haunt her and make her and her ponies pay for her crimes.