Summary of the Betrayal

What may have in another time been a most shocking and foul betrayal of trust, Guilliman the Warmaster cast aside what loyalty he had for the Emperor, Master of Mankind.  Instead all that could be heard was the mocking laughter of a thirsting god as the Warmaster beheld his terrible actions. 

Warmaster Guilliman had gathered close the allies he could be sure of, Horus of the Luna Wolves, Sanguinius of the recently sanctioned Blood Angels, Angron of the World Eaters and Fulgrim of the Emperor's Children. To these Primarchs he set forth his plans.  The Imperium it was clear had failed, having fallen into infighting and relgious zeal was quickly replacing logic and reason, the Emperor was unwilling or unable to do anything about it.  As such a new Empire would be built successor to the Imperium and built upon her original ideals. This new Empire would be Imperium Secundus and would be begun on a distant world known as Isstvan III. 

Image from Forge World's Horus Heresy Book 1 Betrayal.

Gathering their might the 5 Legions traveled to Isstvan III. A system that had recently turned against the failing Imperium, or the Dark Imperium as Guilliman's followers would term it. Sending down those warriors who's loyalty to the Warmaster may be in question, as the initial attack wave the Primarchs argued over the next phase of the battle plan. Guilliman knew what must be done, he could hardly leave a force with the combined might of 2 Legions at his back. Even if they would be effectively stranded on Isstvan III. Finally his brother primarchs convinced the Warmaster to do the unthinkable. With a heavy heart Guilliman gave the order do launch the bombardment to begin. None of this sat well with Angron who at the first opportunity warned the Legionnaires on the surface of what was coming.  As Guilliman learned of this, he could only give a wry smile. At least some of the Legionnaires would receive an honourable death. 

Angron's warning, although only moments before the bombardment struck was sufficient to allow the Legionnaires on the surface time to prepare. Over a hundred thousand marines died in an instant, and yet, as many survived that terrible betrayal. As the survivors clambered from their hiding spots they saw only the contrails of falling drop pods. At the very least they would have a chance for vengeance!

Many legends were made and broken on the scorched and blackened husk of a world that Isstvan III had become. The Secessionists suffered a terrible toll at the hands of the Imperialists but they were ultimately successful. Almost a quarter of a million Legionnaires had fallen on Isstvan III, the equivalent strength of two full Legions.  From this horrific event the Warmaster emerged more determined than ever to make the future worthy of the weight of the past.

Whatever response the Warmaster expected the Dark Imperium's to have, it is likely he never expected that there would be no response what so ever.  After Isstvan, Guilliman quickly secured those worlds owing fealty to him.  With the combined might of the Legions supporting the Warmaster brought almost the entirety of the Ultima Segmentum under the Warmaster's direct control.  Although Isstvan III had taken many months to finally and totally secure the Warmaster still had the chance to take control of the war.

The Imperium had meanwhile fallen further upon itself.  The self styled Paladins facing off against their brother Legions that were pursuing the extremes that the Legions were now permitted to follow under the Emperor's name.  The removal of sanctions placed upon the Mechanicum had split the organisation into two warring factions, aided by The Dark Imperium and Guilliman's Imperium Secundus alike.  A battle that was turning Mars into one of the most hotly contested worlds in the galaxy.  Finally the Khan, Magister Militum of the Imperium seems to have turned from the goal of preparing the Imperium for war against Guilliman in a personal quest for revenge against Corax and his Raven Guard. 

tl:dr

*Scions of Guilliman, World Eaters, Blood Angels, Luna Wolves and Emperor's Children Legions go to Isstvan III in order to purge their Legions of Warriors not dedicated to Guilliman's Imperium Secundus.

*The Secessionists are ultimately successful on Isstvan III where approximately 250,000 Legionnaires died.

*The Imperialist Paladins fought against the other Imperialist forces, reducing their ability to react to the Betrayal on Isstvan.

*The Martian Mechanicum continues its civil war aided by forces from both the Dark Imperium and Imperium Secundus

*The White Scars savagely attack the Raven Guard, where the naval battles swing in the Raven Guard's favour.

*Jagatai Khan primarch of the white Scars and Magister Militum forsakes his duties of preparing the Imperium against Guilliman's Treachery.