A great darkness has befallen the galaxy, and the armies of Chaos are rampant. To survive, humanity must retaliate and take back what they have lost. By the will of the reborn primarch, Roboute Guilliman, is the Indomitus Crusade launched - a military undertaking that eclipses all others in known history. From the Throneworld of Terra does the Avenging Son hurl his fleets, their mission the very salvation of mankind.

As vessels in their thousands burn through the cold void, the attention of Fleetmistress VanLeskus turns to the Machorta Sound - a region under attack by a dreaded Slaughter Host of the Dark Gods. The success of the Indomitus Crusade will be determined by this conflict, and the desperate mission of Battlegroup Saint Aster, led by Space Marine Lieutenant Messinius. Even then it is but a prelude to the forthcoming bloodshed.