If you've read The Journal of the Two Sisters, you'd know that the Crystal Empire was previously ruled by Princess Amore, before King Sombra ruled over it. Also, Starswirl created an age reversal spell and uses it on himself. The Reflections arc in the comic series showed Starswirl and Celestia traveling through the magic mirror, which led to Celestia eventually finding the alternate dimension with Sombra ruling alternate Equestria.