In the immediate aftermath of the Word Bearers' attack on Calth, survivors on both sides of the conflict were forced to take refuge in underground arcologies to escape the radiation of the tormented star Veridia. And while the Primarch of the Ultramarines, Roboute Guilliman, has laid plans for many seemingly unthinkable contingencies, in the underworld of Calth his sons face a war of a whole new nature—whether Steloc Ephon, the famed captain of the "Glorious" 19th Company, can become the one who will lead them to final victory over the traitors? Perhaps, if he can control his burning thirst for revenge...