The Blood Pact arrived on Devos IV in some strength. They were nearly prevented from landing by the vigilance of the Imperial Navy and the Black Templars. But in the event perhaps two BCT sized formations were landed.
The Blood Pact were well aware that theirs was a suicide mission and that they were all condemned men. However, in an unwelcome parallel to the DKK, their fatalistic outlook would make no difference to their performance.
Their tactics were tried and tested; with the city environment precluding large scale armoured warfare, facing over ten Divisions of the Imperial Guard, they went for 'holding on to the enemies belt buckle' when engaged; staying so close that the Imperial Guard's advantage in artillery and close air support.
Their usual modus operandi was to heavily fortify a line in the ruins, using the shells of buildings, rubble and the other parts of the built environment to create positions from which they could fight the Imperial Guard, creating interlocking fields of fire, booby trapping dead ground and using their preparation time wisely. Pressed bodies of other groups in the city, former PDF and Militia could be used as bait or to secure a flank or make a counter attack, but were usually used to dig trenches and otherwise prepare their fighting positions.
They would defend a fighting line on one day, and pull 90% of their strength back overnight to an alternative position tens of meters behind the previous day's. The Imperial Guard would hammer the now lightly defended position, they couldn't not do that. The defences would be subject to mortaring, including coloured smoke to mark the target area for the airstrikes which would be rolling in, bombing runs by CS Marauders and strafing runs by Vultures and Avengers. The following infantry assault would be protected by a rolling barrage by the on call artillery.
The Imperial Guard would take Xyphonica, that was a given. But it would a take a long time and they would pay a heavy price.