One year after the treaty with the Changelings and the attack of the mysterious ponies, Twilight Sparkle and her friends visit Shining Armor and Cadance in the Crystal Empire. She unearths an unknown vault in the library and an otherworldly figure on the other side reveals the long forgotten past of a long forsaken land. After talking to the princesses about it, the sisters Celestia and Luna go into details of their painful past involving an old enemy of the Equestrian Principality.... Who decides to show up sooner than they wish.