Even before he was interred in the golden throne the emperor saw all. In his plans for mankind he paved the path for mankind's future through the stars. His vanguard was intended to the Primarchs. Humanity 2.0, but like humanity some proved unworthy. Some died to see the Emperor's vision protected while others made an even greater sacrifice for mankind's destiny. The Primarchs were never intended to be nothing but warriors to fight and die for the Emperor. They were intended to be the first of a new, better human race. Prototypes for a humanity which would rule the cosmos as the best the galaxy had to offer. In the annals of history two of those great warriors have been expunged from all records. The masses have been lead to believe that the turmoil of the age or perhaps an even greater betrayal than their fallen bothers has led to this. The truth is only known to a select few, among them their fellow Primarchs and the most senior of the high lords. The truth, it was not the greatest of betrayals but the greatest of sacrifices that erased these great warriors.
Following the death of the betrayer and the ascension of the Emperor humanity was in chaos and only decisive action could save it. The Primarchs debated amongst themselves what to do but eventually agreed that for the safety of humanity the numbers of their fighting forces must be limited. As a first step the great legions were vivisected into smaller chapters not to exceed 1000 soldiers strong. They were restructured to serve no single lord so that if one should fall to temptation the rest would not follow. But the two lost Primarchs revealed a fatal flaw in the Emperor's initial plan. They and their legions were female, and decimated though they were, they alone granted the Emperor's new breed of mankind the chance to expand out of control.
Some amongst the Primarch's raged against the proposal. Their sisters, like them, had sacrificed so much and seen firsthand the sorrow of the Emperor's ascension. For their part the battle sisters of the forgotten legions accepted their fate and volunteered willingly. They could not allow humanity to collapse into darkness out of fear of their own protectors. And so they choose one last sacrifice. Knowing they would die the best amongst them hurled themselves after the traitors they had fought and vanished into hell itself. Those few left behind hunted down all who had seen their faces, all who knew their identity. Their very existence became a secret to be taken to the grave. And then when all record and memory had been extinguished they too journeyed into hell.
Though the Primarchs and their surviving warriors would know the truth they too would speak no more. Some would take this knowledge to their graves. Others would follow their most courageous sisters into hell. But none would speak of those who were to be forgotten. And so the fading of memory would claim them that none should know they ever existed.
But the Emperor's plans are never so easily forgotten. The technology to create the sisters was too precious to simply destroy. Instead it was sealed away. Likewise the genetic legacy of the forgotten Primarchs was equally important and was preserved in secret. It was planned that one day the Emperor's vision must come to fruition and when it did the blood of the forgotten Primarchs would be necessary. Recently some amongst the high lords have come to believe those days are coming. As the darkest of days approaches mankind needs beacons of light. And so the Ordo Carnificum, the order of execution, was formed to execute the Emperor's grand vision. Numbered among the Carnificum's ranks is the Scythes of the Emperor, a chapter resurrected from the blood of those long forgotten Primarchs. Given task to execute the Emperor's will they strike deep into the corruption that rots the Empire and none may see their faces and live.