The Siege of Castellax

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 drives his own slaves even 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?