The Chaos Space Marines of the Iron Warriors Legion have long been renowned as the masters of siege warfare, able to inflict devastating firepower and unimaginable cruelty upon their foes. On the world of Castellax, twisted Warsmith Andraaz builds his own empire even as a system-crushing ork Waaagh! approaches, and drives his own slaves ever harder to meet their production quotas and supply materiel for the Legion's many warbands. Their walls are strong and their weapons ready, but how long can the planet hold out against the deadly greenskin invasion when whisperings of rebellion begin to pass between the Iron Warriors' downtrodden vassals?