While most of the sector’s attention was focussed on Tarsis Major, the Ork Warlord Nazghat continued to pursue his campaign in the Mabb Nebula. The commander of Fort Aerin received reinforcements in early 01.009M42, with a Porphyrian and a Praetorian regiment bolstering the defences.
However the Porphyrians were inexperienced and had never fought Orks before. Nazghat’s veteran troops smashed into the front lines, causing chaos in the Porphyrian lines, and massacring over half of the regiment’s troops. The formation was quickly pulled out of the line and replaced with the tougher Praetorian Guard.
By now the Imperium realised that holding onto fort Aerin would be practically impossible. All they could do was hope the Praetorians could hold the line long enough for an evacuation to be effected.