“The Imperium! How mighty its aspect! How far-reaching its boundaries! As one world dies, ten more are brought into the fold. Fear us, for we count the lives of planets, not men!”
-Cardinal Morius Blate, Ecclesiarch Primus
It is the 41st Millennium. The Imperium of Man is hailed as the greatest stellar empire humanity has ever raised up, its extent taking in the length and breadth of the entire galaxy. The worlds upon which Mankind resides vary from factory-hives populated by uncounted billions to feudal cultures ruled by savage, warlike kings. There are no limits to which man does not go to realise his manifest destiny to rule the stars, and he makes his home wherever he may, whether in the clouds, beneath the sea, or in the depths of space itself.
Though humanity long ago learned to travel between the stars, the technological marvels upon which its dominion of the galaxy is founded are long gone. Learning is a thing of the dim and distant past, cast down by generation after generation of war and calamity. No man now knows how many times humanity has soared to the lofty heights of achievement only to plummet once more to the depths of barbarity. The science that caused both advancement and debasement is now the subject of superstition and fear. Only fragments of it are remembered, and then only by rote, passed from one generation of Tech-Priest to the next, each iteration ever more distorted than the last. While most men know how to operate a weapon, they have no inkling of the processes that power it. Only through prayer are the machines Mankind relies upon maintained, and the machine spirits are easily displeased.
The Emperor, or rather the High Lords of Terra who rule in His name, claims the entirety of the galaxy as sovereign domain. The Imperium occupies millions of planets, but in reality Mankind is cast across the vast depths of space and is to be found on more worlds than can be imagined. What long-forgotten tides of war washed human colony vessels up on planets from one end of the galaxy to the other can scarcely be imagined, but some claim there are likely to be more unknown worlds populated by men than there are known worlds in the Imperium.
But it was not always thus. Ten thousand years ago, or so the legends say, the Emperor once walked among his subjects. Having united the disparate tribes of ancient, blasted Terra at the very moment that terrible galaxy-wide Warp storms finally dissipated, he led his legions on the Great Crusade. Moving outwards from Terra, the Emperor and his most trusted and beloved scions cast down alien empires and established contact with human worlds isolated by war and the Warp since time immemorial. This was an age of reason and optimism, where Mankind could still prevail by science and by honour. The Emperor sought not adoration and he rejected the worship of false gods. He promised unity, but it was not to be.
At the height of the Emperor’s crusade he returned to Terra to begin the transition from an era of conquest to one of order. But such an age was never to come, for at the height of the Great Crusade the Emperor’s most trusted son, Horus, turned against him. The ensuing civil war consumed the nascent Imperium, plunging the newly claimed worlds into war and bloodshed of a scale never before seen. The galactic carnage that was the Horus Heresy was only brought to an end when the Emperor faced his son, the Warmaster Horus, at the height of the Siege of Terra. Horus was slain, but at such terrible cost that the Emperor was wounded unto death. Only by his interment in the arcane life-sustaining machineries of the Golden Throne, a wonder of Dark Age technology wrought by the hand of the Emperor himself as if he knew his doom long before, could the Emperor persevere. From that age to this, the Emperor has ruled from the heart of his palace on Terra, though his god-like energies are entirely devoted to steering the soul of Mankind through the subsequent ten millennia of war and calamity. Not a single word has passed his withered lips, and his body is nothing more than a dry, black husk. The Tech-Priests have long ago lost the knowledge to maintain the machine systems of the Golden Throne, if ever they truly had such knowledge. Year by year, one system at a time, the machineries of the Golden Throne fail, yet still the Emperor lives on, a god to the countless teeming billions of subjects cast across the immense void.
The Imperium has endured for ten thousand years, despite the depredations of every manner of foe. Rebels and heretics attack from within, like parasites gnawing upon the flesh of a gargantuan host. Aliens rise up from the black voids in an effort to enslave or consume Mankind. The creatures of the Warp slaver endlessly for mortal souls and countless fools have entered into bargains they shall regret for all eternity. The Age of Imperium still grinds inexorably onwards, the weight of millennia and the unimaginable mass of the Imperium’s institutions nigh unstoppable. The entire edifice is rotten to the core, but its foundations are sunk deep, and entire worlds can fall silent and galaxy-spanning institutions fail, their demise going without comment for decades.
Many among the teeming billions of humanity exist in a permanent state of dystopian barbarism and superstition, their lives regimented through centuries of dogmatic repetition. Few leave the planet of their birth, and many are completely unaware that their own world is but one amongst a million planets. Most live their entire lives in stagnant duty and work to drive the vast engines of war essential for Mankind’s survival. It is only through this that the bulk of humanity is spared death at the hands of one of the many enemies clamouring to extinguish its existence.