Chrysalis returned to defeat Celestia. She forced the alicorns into hiding. Luna, hiding in the Everfree forest, gives birth to two fillies. One, who is as dark a Luna, is named Dusk. The other is fiery red and is named Comet.
Celestia suspects that Comet is part of an important role in the Prophecy of Fire, and ancient prophecy issued many years ago. But Comet must stay hidden until the time is right, or risk the wrath of Queen Chrysalis.
But, one day, Comet is seen by a changeling. An attack from a large swarm reveals a power that, if tamed, can release Equestria from the rule of the Changelings.