Celestia, Luna, and Twilight have been trying to stop the war against the Crystal Empire when Sombra returned and took over the kingdom once again. He had turned Cadence, Shining Armor and their daughter Flurryheart into a crystal statue.

The princesses send you to stop them as they fear the King of Shadows. You have been the only pony known to use and be able to control both light and dark magic so they hoped you could stop him.