In the Pandorax System, on the death world of Pythos, an ancient secret that has laid buried for millennia. Ignorant of this terror, troopers of the Catachan 183rd, stranded on Pythos find themselves under attack by the forces of Chaos. Daemons in their thousands flock the sky, and none other than Warmaster Abaddon leads the attack. With the Death Guard and Black Legion arrayed against them, the Catachans appear to be doomed, until salvation arrives in the form of the entire Dark Angels Chapter. But what is the so-called Damnation Cache'? What secrets does it harbour and why has it also attracted the attention of the daemon-hunting Grey Knights?