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The Galaxy was in Chaos. Beyond the stars and even within their own system so many good people of the Imperium had declared themselves to be the warriors of Horus Lupercal and his rebellion. At first the Praetorian had to try and command his brothers to Istvaan, where he soon learned of their own Slaughter. After that, through the actions of Alpharius, the Astropathing Choir which received messages upon Pluto was silenced permanently.
Rogal Dorn had killed his brother for that action. Alpharius’s blood was upon the hands of Rogal Dorn. Just like the traitors upon Istvaan he had killed those he would have considered a comrade in arms. Sure he might not have liked Alpharius but that did not mean he needed to claim the life of his brother.
After that Dorn had worked tirelessly to try and gain some semblance of the Galaxy. Every minute that went by he wondered if the war had been won by his loyal brothers or if Horus was simply one step away from claiming the Galaxy. That he simply slaughtered the rest of the nine loyal Legions and was leaving Rogal Dorn for last pickings.
Those were musings and unrealistic. Rogal Dorn knew Horus Lupercal and during his stay upon Terra studied every piece of literature that existed about the Primarch. Even before that though Dorn would tell you that Horus Lupercal did not play with his prey like the Night Haunter or Fulgrim.
Horus was going right for Terra. The time between the Istvaan Massacre and his possible arrival was the Warmaster simply gaining the overwhelming strength that he needed. By isolating Terra like he had cut off whatever higher echelon of communication Rogal Dorn might have had in organizing the resistance.
If Rogal had not allowed himself to be tricked and had given the orders that got so many of his brothers killed…
Rogal Dorn stopped that line of thought and settled on what was a true fact. The only brother he had killed was Alpharius. All of the others had been killed by Horus Lupercal and those who fained the face of loyalty.
A lesson learned Rogal Dorn decided that he will only trust those he knows he can trust. Even those loyal, like Leman Russ, simply threw themselves at the problem instead of working with Rogal to fix the issue. He could not get the Wolf King to see logic, so he simply decided to count his beloved brother as a liability.
Thus he needed those he could truly trust by his side. Those were Sanguinius, Roboute, and lastly TalOS.
Such was the reason he was now being lowered in a Thunderhawk down to Mars, too what was a siege camp surrounding the Magma City.