Cadance is ill.

Twilight Sparkle took to wing and flew to the Crystal Empire with only those three words, but did not find anything there as she expected it to be. With no answers available and no remedy known, Celestia calls upon an old ally for help, and Twilight finds herself rapidly drawn into a terrifying new world that will challenge what she knows of Equestria, of dreams, and of her role on the vast, cosmic stage of reality.