The final nail in the coffin at Montgate came on 2711.008M42 when the final battle for the city began. The Catechism employed their elite Legions as well as summoning another horde of daemons to complete the destruction of Corchorans North Merrin Army.
The combined power of the Traitor Legions and their daemonic allies soon swept aside any lingering resistance from the shattered remnants of Corchoran's Librian, Zadocian and Tarsisian divisions, but the real fighting erupted as a large force of Daemonhunters, recently arrived having heard of the daemonic incursions on Merrin, engaged with the main force of the Catechism's strength.
Once engaged the second part of the Imperial plan came into play as once again the Ultramarines utilised their drop pod capability, arriving right at the heart of the action. As 2811.008M42 began ans the Catechsims' cultists prepared to move into the city proper, on the outskirts the violent battle between the Daemonhunters and the Catechism continued without pause. The Ultramarines were initially successful following their drop, but gradually the Traitor Legions and the daemonic horde began to gain the upper hand. Late on 3011.008M42, just minutes before the daemons faded back into the work, the Imperial resolve fell and the Catechism's troops managed to outflank the Imperial positions, forcing the Daemonhunters, minus several good men including an inquisitor, and the Ultramarines, similarly battered, to abandon the city of Montgate to its fate.
Thousands of Guard died and those that were captured quickly wished they had too.