Umbra. Little is known about that race, long ago entombed in the arctic ice. For millennia they had been dormant. Dead, as the few ponies who knew about them assumed.

But now ponies are disappearing from the Crystal Empire, all of them haunted by the same eerie dream. Twilight is tasked with researching the umbra to find an effective defense against their sinister magic, but her discoveries set into motion a chain of events that threatens much more than Equestria.