The forces of the Order have been seen heavily in the Armageddon sector. They protect lone worlds from attack and strongly regulate the lives of those under their jurisdiction, and that of the Ecclesiarchy. Their power comes from the great Sanctorum of St. Katherine which is built within the high walls of Hive Tempestora which is also a centre for Ecclesiarchal activities on Armageddon.
Their positions were assaulted by Orks emerging from vast submersibles emerging from the Boiling Sea. The Sisters were the front line defence of Hive Tempestora but due to the sheer surprise and ferocity of the Ork attack, many units were caught unaware and floundered. This allowed the Orks to take control of the majority of the underhive within a few hours of landing.
The Sisters were gradually pushed back through the dark, dank tunnels of the Hive with each successive wave of Orks. When the Orks reached the heart of the Hive, the Imperial command structure began to break down as the hordes rapidly pushed on. Several units were dispatched to defend area which had already been overrun. Their resistance quickly became a running battle for the Sisters and what few Hive militia they could rally while the Orks revelled in the chaos they were causing and continued to push the Sisters back to the great armoured doors of the Sanctorum.
Here, the Sisters refused to retreat any further. The battle intensified as the Order rallied against the Orks, calling for every member of the Ecclesiarchy present to lay down their lives in defence of the holy site. The Orders numbers began to dwindle as Ork and Sister died together. Through sheer weight of numbers the Orks gradually overwhelmed the Sisters and pushed them back. Finally, one last charge by Warboss Nargrim and his mega-armoured Nobz caused the Sister's line to crumble.
The following period became one of the darkest in the history of the Order. The lines of Sisters, broken and scattered, attempted to form up again but were readily cut down by the huge tide of Orks. Finally, after an hour of savage fighting, no humans were left alive. The Orks had taken Hive Tempestora and the Sanctorum.
After this massive battle the Order could barely field three companies. The loss of their Sanctorum spread throughout the Order and many Novices questioned their faith in the Emperor. Though horribly damaged, the Order recovered quickly and swore to make the Orks pay for damaging their holy site and would retake Tempestora at all costs. Before they could do this however, their depleted numbers were spread over the rest of the planet. Here they were used to solidify the defences of key areas and were pivotal in keeping the Orks from overruning several areas.