A long time has passed since Princess Twilight ascended. Her friends have grown into historical figures and heroes, harbingers of a peaceful era. Magic flourishes, with the elder mages teaching new apprentices.
One such filly learns the principles of magic from a wizard who has... unorthodox methods. He pulls stories from the journal of an ancient and powerful mage, teaching through the critical points in his life. She is immersed in the tales through powerful magic, able to see the events unfold with amazing clarity.
Something in the spell reacts to her, though - bonding to her in profound ways. She can interact with the protagonist to change events. What she doesn't know is that her teacher and these "stories" are not as clear-cut or simple as they appear. Her actions will change Equestria forever.