Let us begin with the discovery of the Steel Tablet on Pluto. It was the Imperial Conquistadors that discovered and disturbed the Steel Tablet thus releasing the Dark Symmetry into the universe of mankind. But this cannot be seen as an accident, this was no chance discovery. Besides the natural curiosity of man there was something else at work, a hidden force acting on the Conquistadors like the lure of a serpent. When the doomed Conquistador touched the Steel Tablet it was like the toiling of a mighty bell, a resonance going out into the void but it was also a sign, an omen declaring that the fruit of man's soul was ripe. Swollen by greed, ambition and betrayal the human race had finally proved itself a worthy sacrifice to the Dark Soul. The Dark Symmetry spread throughout our reality warping it to the purpose of the Dark Soul and rendering a large part of human technology so maliciously unreliable that to use it was to court disaster.
The Dark Symmetry was only the vanguard, the precursor of the arrival of the Dark Legion itself. In much the same way as mankind altered entire worlds to accommodate his presence, so the Dark Symmetry worked upon the reality of mankind to accommodate the presence of the Dark Soul.
Some fifty years later, when the planet Nero was discovered its significance was not recognized, though many were filled with an unreasonable sense of foreboding by its sudden appearance and disturbing presence. The first recorded awareness of the Dark Soul's existence came from the speeches of a young political and religious activist by the name of Nathaniel Durand. He was the first person to bring the imminent threat of the Dark Legion to public attention. But it wasn't until some fifty years after his inauguration as Cardinal of the Brotherhood that the Dark Legion launched its first massive assault.
The physical presence of the Dark Legion was released into the solar system after the ever-inquisitive Imperial Conquistadors broke the First Seal of Repulsion on the planet Nero. In the year 51 Y.C. the Dark Legion launched a massive attack across the solar system raising up a host of Dark Citadels and bringing new levels of terror to the battlefields of mankind.
It was only the Cardinal's vision and strength of faith that saved mankind from complete annihilation. Leading a combined human force he defeated the Dark Legion during the Venusian Crusade and their presence began to withdraw from the solar system. A brief show of strength by Algeroth's Nepharite Overlord Saladin on Mars, reminded humanity that the Dark Legion was not completely vanquished but had simply retreated, biding their time before the next and final assault on the soul of mankind.
Now after almost twelve hundred years of physical absence they have returned. The Dark Legion is back to finish the task they began so long ago and as the forces of mankind struggle in the grip of the Second Corporate Wars there seems little hope that they shall survive this, the ultimate attempt of their worlds, their lives, their very soul.