The past always comes back to haunt the present, there is no escaping the inevitable. For decades, Celestia has kept a secret from her student. A secret she made out of pain, and through pain will be revealed for all to see; for Twilight Sparkle is
Five hundred years ago Celestia's neice was born. Since then Celestia has sought out proper foster parents for the unique foal. All of that has now changed. Celestia and Twilight are on their own.
Twilight is unable to control her powers during the entrance exam to the School for Talented Unicorns, and in order to reduce the damage caused by her power surges, Celestia shrinks her down until she would later learn how to control her powers.