Twilight Sparkle has a secret: her mind is not hers alone.
King Sombra, tyrant of the Crystal Empire, was spared from his destruction. Twilight Sparkle welcomed his severed consciousness into her own, and began to learn from and hopefully teach the fallen unicorn. No one knows what she has done, and now fate has thrown a wrench into her plans.
Back in Ponyville after her coronation as Equestria's newest princess, Sombra refuses to ignore Twilight's new station in life.