A penal colony for the Prastian straits, Eren is a world set aside for hard labour. The refuse of Claudus has been shipped to its sister for millennia, creating a stinking mass of industrial spoil, human effluent and mass graves. In amongst these landfills are hundreds of prison camps, where convicts toil to reclaim useful components from the oceans of waste for recycling by their Mechanicus masters. The atmosphere of Eren is toxic over prolonged exposure, due to both its natural make up and the poisons released by the pollutants dumped there, and life is short for the prisoners.

The insurrection on Claudus has seen tens of thousands of new inmates brought to Eren, but rather than accept their fate, a massed uprising against the Adeptus Mechanicus occurred two months previously, and now the inmates control over half of the prison complexes. They aim to seize the one space port, Tyrrell's Landing so that they can return to Claudus to renew their war. Between them and their goal stand a newly reinforced cadre of Mechanicus tech guard, and the toxic environs of Eren itself.