With the abilities of the Astartes, climbing the suspect Emperor's Icon would be no problem, but they would inevitably damage that magnificent monument. They would have to gouge footholds into the stone in front of them, for instance. Once the Astartes had finished climbing it, wouldn't the Emperor's Icon be riddled with holes?
"My lord, the Astra Militarum's Proud Moon above us has sent a transmission asking what assistance they can provide."
"Tell them to scan this Emperor's Icon with their Auspex, at the highest resolution. Say it's an order from the Astartes."
"Yes, my lord."
A full-power scan would take a long time. Frosa's Astra Militarum troops nearby had already begun rounding up the people. Their manpower was plainly nowhere near sufficient, so they could only coax and deceive them, claiming that the Ecclesiarchy was coming to conduct a grand ceremony. If they were sufficiently faithful, they should gather at designated locations and worship until the Ecclesiarchy's missionaries finished their rites.
Most people went to the designated areas. Many still harbored doubts, but the moment they saw the Astartes, all their concerns vanished.
My Emperor above, even the Astartes have come! This must be an incredibly important ceremony. Coming here was definitely the right choice!
The Astartes swiftly ordered the surrounding troops to establish defensive lines where they stood. If the problem really lay with the Emperor's Icon, there was no telling how many people had worshipped it over the years. Who knew how many had already been converted into cultists?
As the troops under the Astartes' command surrounded the Emperor's Icon, the enemy also noticed those "skulls that would delight the Brass Throne beyond measure."
The Khorne Cultists did not attack immediately. They gathered the other cultists roaming the surrounding desert, preparing to overwhelm the enemy defenses in