“On the longest day of the thousandth year, the stars will aid in her escape.” Celestia was prepared to face Nightmare Moon in battle. What she wasn't prepared for was to face her in a hospital bed.
When Twilight Sparkle was killed in the changeling invasion all of Equestria mourned. But when a mysterious figure in armor slays Chrysalis in front of the princesses they will learn that anyone can change. For better or worse.