And so it came to pass that the legions of those who had declared for the accursed Apostate Cardinal gathered on the benighted world of Rahe, and before the assembled might of his heretical army, perhaps enough to trouble even the High Lords of Terra, the Apostate spewed his vile heresies, standing atop one of Rahe's fabled temple ziggarauts. At the climax of his speech, just as the crowd was roaring its loudest, a single shot rang out and a single bullet sheared through half a dozen conversion fields and ended the Apostate's life mid sentence
The heretics were already in disarray as the first drop pods rained down. Traitors died by the hundred under the bolters of the Astartes, and beacons were planted and orbital defence systems disabled and the golden temple ziggarauts of Rahe were drowned in nuclear fire. The last ones to leave the dying world was an Astarte scout detail, tasked by the High Lords themselves to recover the archeotech rifle responsible for the death of the Archeretic, the assassin himself was more than expendable. An unassuming object, the rifle, little more than a long black cylinder, with a single credo inscribed over one portion:
Ready for scope work!
Scoped and dropped!