"Mankind stands upon the brink; on the one hand lies a realm of unimaginable power, on the other awaits darkness, death and utter damnation. Only those that follow the guiding light of the Emperor may save their souls."
— Inquisitor Damarn, Ordo Malleus
The Imperium of Man at the end of the 41st Millennium is a dystopic society by the standards of the men and women of the 21st Century. The unexpected horrors of the Horus Heresy fatally weakened the nascent Imperium, but more importantly, starting with the Emperor himself, claimed some of its best warriors, technocrats, administrators and diplomats - who fell either in battle with the Traitor Legions or were corrupted by Chaos themselves. But perhaps the most significant consequence seems to have been that like the Emperor, the Imperium itself entered a slowly decaying stasis, while Chaos and the Imperium's myriad xenos enemies are ascendant.
It is known that the period immediately following the Heresy was one of near-anarchy, and that the continuity of the Imperium was not assured. Roboute Guilliman, the Primarch of the Ultramarines, was one of those credited with taking decisive action that kept the Imperium together, possibly with the help of other Loyalist forces and of newly-created Imperial organisations such as the Inquisition and the Officio Assassinorum (it is known that at least one Assassin Temple, the Callidus, was already operating with Imperial sanction, and had successfully assassinated Konrad Curze, the Primarch of the Night Lords Traitor Legion). Ironically, while the Imperium survived, it also seems to be fulfilling predictions or prophesies made about its future by a variety of Chaotic enemies, Traitors, and more neutral or potentially sympathetic observers like the Eldar. Supreme among such ironic twists is the deification of the Emperor and the attendant creation of the Ecclesiarchy, the galaxy-wide church dedicated to the Imperial Creed that teaches that the Emperor is the God of Mankind. The Emperor's express purpose and one of the pillars of the pre-Heresy Imperial doctrine known as the Imperial Truth that was spread by the Imperial forces during the Great Crusade was the elimination of superstition and of belief in supernatural powers, gods, and religion and the promotion of reason and science. It is worth noting that among the ploys used by the Ruinous Powers to turn Horus to their service were uncannily accurate visions of the post-Heresy Imperium in which the Emperor and his Loyalist Primarchs were worshiped by the Imperial masses as a God and his saints, respectively. Of course, unknown to Horus, this was a future caused by his actions, not prevented by them.
Another measure of the Imperium's stagnation has been the lack of real technological progress over the last 10,000 Terran years and a fear of unknown or ancient technology that sometimes borders on irrational superstition. It seems that the last major era of human technological advancement was the era of discoveries and technological applications made during the planning and execution of the Great Crusade.
In short, the Imperium has become a repressive regime marked by extreme levels of superstition, political repression, religious intolerance, bureaucratisation, economic stagnation, technological regression and inequality. Corruption and injustice are rampant and human life is increasingly worth very little in a galaxy teeming with literally trillions of people. But though the Imperium is repressive and stagnant, it is also the only thing holding the enemies of Mankind at bay. Many in the highest levels of the Imperium know that it must reform if it is to survive, but they fear that the cure might very well kill the patient. And in small pockets of the Imperium there are men and women willing to shoulder the burden of making their small corner of the universe a better place for all by shouldering the responsibilities and the burdens of heroes.
But recently, in 999.M41, the Adeptus Mechanicus has secretly reported to the High Lords of Terra that in addition to all the other problems besetting the Imperium, the mechanisms of the Golden Throne are malfunctioning and they no longer possess the necessary knowledge to repair them. Whether the nobles of the Imperium like it or not, a terrible change is coming...