The Primarch's Sword is a Strike Cruiser in the Ultramarines Chapter that took part in the Indomitus Crusade. In its service to the Crusade, it was commanded by shipmaster Aethor and Primaris Lieutenant Cassian as he led his forces to victory against the flesh-cults that had overrun the Imperium world Knossa. After order was restored to Knossa, Cassian ordered the Primarch's Sword to return to the main forces of the Indomitus Crusade, but disaster struck when the Strike Cruiser was hit by a Warpstorm. Though the Primarch's Sword eventually escaped from the storm, it was left heavily damaged and the Ultramarines found themselves stranded in the Kalides System. With the Strike Cruiser left unable to enter the Warp, Cassian consulted with Chaplain Dematris and Librarian Keritraeus on their best course of action. After much consideration, it was decided they would go to the nearby Imperium world Kalides Prime, so they could repair the Primarch's Sword and rejoin the Indomitus Crusade.