House Tourmaline has been banished from Equestria for over three-hundred years. When the kingdom was young and Princess Celestia and Princess Luna's rule was all but secure, the unicorn Sword Play used dark magic to transform himself and his sons and daughters into alicorns, hoping their wings and horns would make them contenders to the throne of Equestria. Fearful that Sword Play's powerful mastery of magic would unhinge the peace that the two of them had fought so hard to maintain, Celestia and Luna banished the powerful unicorn and his family from Equestria, naming them as the "Curséd Ones" and forbidding them from returning to the kingdom. Three hundred years later, the only surviving daughter of Sword Play, the self-styled Queen Coronet, is plotting her revenge—and her mastery of the dark magic her father used to transform himself into royalty could place herself and her son and daughter upon the Equestrian throne.

Princess Celestia and Princess Luna can feel the threat of Coronet and her offspring looming over them like a dark cloud, and they fear that the Queen's powers have grown too strong to contend with alone. After receiving a veiled threat from Coronet during Hearth's Eve, the two princesses place Equestria under lock-down; forbidding any pony to enter or leave Equestria, they call the Royal Guard and place the most powerful unicorns on all edges of the kingdom, using their magic to cover Equestria in a spell that would prevent any pony from breaking through. Yet Coronet's powers only grow stronger, and her dark magic is beginning to take hold of many mares and stallions, taking advantage of their weak hearts and using them as pawns in her game of power.

While the two princesses and their enemy grapple amongst one another, a young unicorn mare named Gratia is introduced. Although she is nearly an adult, Gratia does not yet possess her cutie mark. Despite her lack of a cutie mark, Gratia is filled with untapped magical energy—an energy that both the Equestrian princesses and the false queen desire to use to meet their own ends. Will Gratia use her magic to save Equestria, or will Queen Coronet's overwhelming influence draw her towards the darkness?

Follow Gratia, Prince Dirk, Princess Quiver, Queen Coronet, Princess Celestia, and Princess Luna in this epic MLP:FiM fantasy adventure!