With Sunset Shimmer returned to Equestria, the mirror portal she used is left in curious position, and it is decided by Princess Cadance and her husband to destroy it, in order to prevent anypony from abusing its power. But, the mirror is not simply a thing of crystal glass; it is a focal point for the world's ambient magic, a great teleportation spell capable of crossing the fabric of reality. With this focus broken, the magic lashes out, and takes with it two ponies, two very important ponies, and leaves the Crystal Empire in chaos while they are missing.

What's worse, wherever it is they've ended up, they didn't arrive together.