With the terrible Black Rainbow now fully unleashed over the darkened skies of Oz, Mombi the Malicious moves to enact the next stage of her nefarious plot: the summoning of a great army of vengeful beasts known as the Phiastons! Long exiled from the kingdom, they venture through the great sky realms with a singular agenda: to destroy the Emerald City and unseat its long-standing leaders. Anticipating the invasion, Glinda, Ozma and Dorothy rally friends, fighters, old adversaries, and an army from the sky to defend Oz, all while sinister plots begin to work their way into the hearts and minds of the heroic Ozians and rebellion threatens to weaken the city's defenses. Terrible secrets and startling revelations come to a head in a climactic battle between Glinda and Mombi that will test loyalties, alter the course of well-known characters, and ultimately determine the very fate of Oz's future.