At the dawn of the Age of the Imperium the Emperor created twenty beings of great power to be his generals in the Great Crusade that would reunite humanity and reclaim the galaxy for man. They were the Primarchs. From their genetic stock the twenty first Space Marine legions were created. With this army of super-humans the Emperor reached out from Terra and none could stand before his might and glory. But then, at the height of the Great Crusade the Emperor abandoned his warriors and retreated to Earth, leaving Horus as Warmaster. Saddened, yet elated at the same time Horus did not question his father's motives, but resumed the Crusade with renewed vigor. He pushed the forces of the Crusade harder and further than Father ever had, reclaiming lost worlds and destroying xenos civilizations at an unprecedented rate. Hoping that the Emperor would take notice and be proud of his children. And the Emperor did finally take notice, when the legions were all but spent, facing nothing but the great unknown of the Halo stars. Then he did finally notice. He congratulated his children with a job well done, then bade them stand down the legions and return to Holy Terra. The war was over, now it was time to return home to rest and bask in the praise of the masses.

But they were all being deceived. The Emperor had never intended for the Great Crusade to set humanity free of all bonds and shackles. He had only intended for it to replace a thousand horrors with one tyranny and a million kings with one tyrant. The Great Crusade was not a great quest to bring freedom, illumination and equality to all - it was only meant to enslave humanity to the will of one man, the Emperor, father to Horus and the other Primarchs. Horus' great success has shown him that there was only one threat to his hegemony - his own sons. If his rule was to be undisputed he must be rid of the Primarchs and the legions. Horus was no man's fool though, not even before the glory of the Emperor. He, and he alone saw through the lies and the deception to discern the Emperor's true motivations - to rule the galaxy absolutely and for all time, the greatest tyrant that ever would be! Knowing full well that he would die in the attempt Horus nevertheless raised his banner and stood up to the Great Enslaver. Great was his joy when fully half his brothers flocked to his side and many other warriors beside. But great was his sadness also, for he knew that many, if not all would die in the battles to come. And he also knew that the chance of victory was slim - he would have to strike hard and fast directly for Terra, sacrificing whatever was needed to get within striking distance of his father. One blow to strike off the head of the monster, or die trying...

So began what the priests of the Corpse-God call the 'Horus Heresy'