Long ago, the Dark Interlopers, unicorns capable of wielding dark magic were cast from the realm of Equestria and banished to another, sealed by powerful magic. However, the seal, after centuries of weakening, has broken and the unicorns, thirsting for revenge, return to claim the years and land that were stolen from them.
Meanwhile, Twilight Sparkle, now in her new castle in Ponyville, still has questions even after her battle with Tirek and now her worth will be tested as she must go toe to toe with an enemy with an ability even Princess Celestia does not have: time travel.