They say the world used to turn. They say that night would follow day in an endless dance. They say that dawn rose, dusk fell, and we worshiped both sun and stars. That was a long time ago. The dance has died. The world has fallen still. We float through the heavens, one half always in light, one half always in shadow. Like the moth of our forests, one wing white and the other black, we are torn. My people are the fortunate. We live in daylight, blessed in the warmth of the sun. Yet across the line, the others lurk in eternal night, afraid... and alone in the dark. I was born in the light. I was sent into darkness. This is my story.

The protagonist, Marshal Kael of the Black Templars, is given a mission by a member of the Ordo Malleus, a daemonhunter named Lady Ymira. He must join her in the destruction of a Chapter of Space Marines, the Knights Eternal, who have been declared Excommunicate Traitoris. She believes that the Chapter Master, Sven Baliik, has accepted the gifts of one of the Chaos Gods, and will soon be granted daemonhood. Kael chooses several Initiates and a Sword Brother from his men, and descends upon Requiem, the homeworld of the Knights Eternal. His goal is to purge the heretic, to kill one he used to call brother, but most of all: Kael is coming to destroy the unholy in the Emperor's name.

A new challenge appears, a new battle begins. Once again the Rider of Legend must fight to save the lands, only this time she finds herself five hundred years in the future.