Corchoran new he would be hard pressed to hold the Traitor Marines but was lucky enough to have the Captain of the Ultramarines 1st company on hand. The Ultramarines had arrived at the beginning of the crisis and had lent nearly half the chapter to the defence of Tarsis, since the planet lay on a direct route to Ultramar itself. With Corchoran's consent the Ultramarine captain agreed a moderately ambitious plan to use the Marine Strike cruiser to drop troops to the rear of the Catechism, catching them between the Adeptus Astartes and Corchoran's troops.
A violent firefight immediately erupted before devolving into a swirling close quarters melee. Loyal and traitor superhumans were loked in vicious hand to hand fighting so confused, that Corchoran's Guard units realised they could do nothing to help for fear of shelling their own side. Late on 2411.008M42 it became apparent that the Ultramarines had lost the brutal fight, and the Catechism were underway again. Seeing that the plan had come to nothing Corchoran began to retreat some 150 miles to the outskirts of Montgate, and prepared for a siege.
Worse was to follow. Overnight on the 27th Catechism troops were observed performing a strange and disturbing ritual to the south and west of the city. the next day more daemons than had previously been encountered assaulted and massacred the Imperial defenders. Space Marine forces were called in and held the creatures of the warp off for several hours but mounting casualties forced even the superhumans to retreat.
The resulting carnage only grew in intensity as the day wore on. This time, the Catechsim forces, due to take up positions in recently assaulted sectors, were unable to take their stations as marauding daemons rampaged entirely out of control. Suburban areas to the south-east of Montgate suffered appallingly and were set ablaze by uncontrolled energies. Walls melted and boiled and other aberrant obsceneties were observed taking place.
The Inquisition even considered ordering an exterminatus on the entire world, such was the depravity and unnatural wanton desctruction in the area.Fortunately the carnage only continued for one day and the daemons vanished. What they left behind this time however was no territory to be taken. The entire region of South East Montgate was gone, razed to the ground and rendered to a cloying stinking ash blowing in the wind. For the first time the full horror of a daemonic incursion had been witnessed and a new word passed into the lexicon of the people of Merrin. "Burnanated".