Twilight Sparkle learned what Celestia was training her for at a young age and she spent every waking moment preparing for it. She was to be used for the purification of Nightmare Moon in order to return Princess Luna to Celestia's side. However on the day, it was all supposed to take place, Celestia was unable to ask for such a sacrifice from Princess Twilight. This fact only hardened Twilight's resolve to reply Celestia for every kindness she had done for her in her short mortal life.