The Fall of Cadia - Robert Rath

Cadia licks its wounds in the wake of the Thirteenth Black Crusade. The heretic forces retreat on all fronts. The day is won. But Lord Castellan Creed cannot rest easy. Something tells him the assault was a mere prelude to something greater, something more final. He is right. Out of the Eye of Terror comes Abaddon the Despoiler, at the head of a warhost unmatched in scale since the dread days of the Horus Heresy.
In the face of the looming apocalypse, Creed must weld the champions of Cadia into a bulwark capable of withstanding Abaddon's fury. And in orbit, the Despoiler himself finds his own alliance teetering on a knife edge...
This is a tale told at epic scale, from the tables of high command to the slaughter of the pylon fields, and with a huge cast of characters from self-styled demigods to the rank-and-file foot soldiers of the Imperium.
This is the story of Abaddon's greatest conquest. This is Cadia's last stand.