As 12.010M42 began the campaign on Bastien increased in intensity as both sides sought to achieve final victory. Both the forces of chaos and the Imperium wanted to finish off Bastien and turn their attention, at least temporarily, to the emergent Necron threat. Both sides launched major offensives in early 12.010M42, with the Imperium concentrating their forces in the south, while the Covenant of Damnation focussed all their attention on taking back Imperious, or Heretic, in the north.
The initial Chaos assault took the Imperium by surprise. The main forces of Veers' crusade, including the Librian regiments, were stationed outside the city, and only a company of Space Marines known as "the Watch", and some reserve forces now guarded the psychologically significant, though strategically now worthless ruins of the city.
The traitor guard attack was so ferocious that within a day the Covenant had surrounded the marines sheltering in the ruins, and quickly oevrwhelmed them. Veers, embarrassed once again by the loss of the city, immediately responded, sending the Librian Guard into the ruined streets. When the loyalist troops met the traitors, the battle was savage, but the largely infantry based Covenant forces suffered badly in the now mostly flattened settlement. Leman Russ tanks and veteran guardsman mounted in chimeras, supported by disciplined ranks of infantry, mowed down the traitor infantry in droves as they pressed home futile counter attacks. By 0412.010M42 the Imperium had retaken the city once more.
The Imperial hold on Nova Imperious remained precarious however, as late on 0612.010M42 Imperial forces including the Red Angels chapter of space marines were outflanked, and seriously beaten just to the north of the city. The Librians and other guard regiments now holding the city remained in a dangerously surrounded salient of Imperial control.