Hidden deep within Canterlot Castle is one of the three most powerful objects ever created, the elements of harmony, the alicorn amulet, and the mirror of Alternet.
The mirror of Alternet shows the viewer different way in which thier life could have turned out.
Twilight Sparkle has recently become princess, and Celestia has decided she is ready to see the mirror. Each day she sees a different possible life she could have lead.