The Imperium of Man is a galaxy-spanning interstellar human empire, the ultimate authority for the majority of the human race in the Delta Palioxis Galaxy in the 41st Millennium CE.
The Imperium was founded by the Emperor of Mankind, also called "The God-Emperor" and the "Master of Mankind", at the end of the Age of Strife, in the 30th Millennium CE. The Emperor, an immortal being born around the year 8,000 BCE in central Anatolia on Old Earth, was the collective reincarnation of all the human shamans who possessed true psychic abilities during the Paleolithic and Neolithic Ages. Imbued with their combined psychic, physical and intellectual power, the Emperor was born an immortal being of unparalleled physical strength, psychic ability, charisma and intellect. The Emperor helped humanity as a whole survive and prosper through the long millennia. In various eras of human history He intervened through various personas, some of them well-known historical personages, to guide Mankind, though such interventions were always brief and shrouded in legend and historical mystery. At the end of the Age of Strife the man who would become the Emperor finally took a direct, public, and permanent role in shaping the future of humanity, believing that the damage done to the human race by 5,000 years of terror, isolation and violence could not be reversed unless He openly guided humanity as its leader. As such, He shed all His prior identities and simply revealed Himself in the 30th Millennium on Terra as the Emperor of Mankind, determined to unite the entire species under His stern but benevolent rule. He intended to replace superstition, fear, poverty and intolerance with reason, science, progress and hope.
The Age of Strife, also sometimes called Old Night, was a 5,000-year-long period of anarchy, war, and destructive technological regression from the 25th to the 30th Millenniums that brought Mankind to the brink of extinction -- and reversed the highly advanced scientific and technological discoveries made by the galaxy-spanning interstellar human civilisation of the Dark Age of Technology that had preceded it. The Age of Strife was caused by the negative psychic and physical effects of the vicious Warp Storms that ravaged large portions of the galaxy beginning in the 25th Millennium, which were the "ripples" in realspace of the psychic gestation of the new Chaos God Slaanesh in the Immaterium. Slaanesh was forming within the Warp as a result of the growing hedonism and utter excess of the ancient Eldar empire in the millennia before its Fall. One result of the Warp Storms was the impossibility of faster-than-light travel or telepathic interstellar communication through the ravaged currents of the Warp, which led to the subsequent isolation of human colony worlds and star systems separated by multi-light-year distances. Human civilisation in the colonised portions of the galaxy fragmented, and where it survived, it took on wildly different forms. On Earth, the planetary economy collapsed with the loss of interstellar trade and communication with the rest of humanity even as the social order broke down. Wars, rebellion, and disease were rampant. Vast swathes of the advanced scientific and technical knowledge accumulated in the previous millennia was lost or forgotten, and a slow descent into pre-industrial barbarism seemed inevitable. Brutal warlords emerged, who carved out new kingdoms to serve as their fiefdoms. The emergence of these often brutal neo-feudal kingdoms, the new nations of the people later termed "techno-barbarians" by Imperial historians, imposed an uneasy peace on the Earth that was frequently interrupted by feudal conflicts. The human race seemed on the brink of extinction at the start of the 30th Millennium, when the man who would become known as the Emperor of Mankind once more inserted Himself into the affairs of humanity.
The Emperor sensed that the Warp Storms that had marked the Age of Strife were starting to subside, and that this offered an opportunity for Mankind to reunite in pursuit of a new Golden Age. The Emperor first emerged from his secret fortress beneath the Himalazian (Himalayan) Mountains in the mid-30th Millennium to unify all the techno-barbarian states of ancient Terra under His rule through either diplomacy or the waging of the brutal military campaigns later called the Unification Wars against the techno-barbarians tribes that ruled vast swathes of the Earth's blighted surface. These wars were waged with superhuman soldiers later known as the Thunder Warriors who had been genetically-engineered by the Emperor's genius to be faster and stronger than base-line humans. Such warriors would provide the early foundation from which He would later decide to create the Space Marines.
After conquering the last of the independent techno-barbarian states of Old Earth, the Emperor began to lay the foundations of His burgeoning Imperium. Understanding that no one man, even one such as He, could rule alone, the Emperor formed His War Council, comprised of his most able generals and a number of high-ranking administrators, the most formidable of whom was Malcador the Sigillite. Malcador was not a warrior, but a man of learning with the bearing of a priest. His origins were unknown to all save perhaps the Emperor Himself, to whom some believed Malcador was distantly related. Malcador was appointed to administer the newly-constructed Imperial Palace and Court in the Himalazian Mountains, and through this appointment also governed newly conquered Terra as his master's left hand. Where Terra had been a place of unending war it now became a place of unceasing activity, production and planning.Just as the conquest of Old Earth was complete a mighty and unforeseen cosmic event occurred. A massive shock wave blasted across the Immaterium, clearing the Warp Storms that had plunged the galaxy into tumult and raged for more than five thousand standard years. It seemed to some to be divine providence, fuelling the beliefs of those that considered the Emperor to be Himself divine (no matter how much He decried this claim). The way to the galaxy was now open and the Emperor's armies would be able to take to the stars, with the other planets of Terra's solar system the first step upon that road.
After conquering the last of the independent techno-barbarian states of Terra, the Emperor secured the allegiance of the Cult Mechanicus of Mars who controlled the most advanced remaining industrial fabrication and scientific research facilities in the human-settled galaxy. The Emperor promised in the Treaty of Mars to protect the Tech-priests religion and respect the sovereignty of the Mechanicum and their Forge Worlds across the galaxy, affording them a level of independence unequalled within the Imperium. Furthermore the Emperor gave to the service of the Mechanicum six of the Houses of Navigators, so that their ships might once again travel safely through the Warp. The powerful Fabricator-General of Mars was given a seat on the War Council of the Great Crusade. With access to the giant manufactoria of Mars, this enabled the Emperor to vastly increase the power of his Legions with improved wargear and supply. In addition, the Tech-priests of Mars lent their arts to the construction of the massive Warp-capable Battleships that could transport the Emperor's Legions across the galaxy, and provided the mighty city-crushing war machines known as the Titans to the ever-expanding Imperial military.The Treaty of Mars married the Terran military might of the Emperor with the industrial strength of Mars and the Mechanicum. Now possessed of the needed manpower and materiel to make His dream a reality, the Emperor mobilised the resources of Terra and Mars to launch a vast new military campaign intended to reunite the whole of Mankind, dispersed across the galaxy, under his rule. This campaign, the Great Crusade, marked the beginning of the period that would later be known as the Age of the Imperium. A vast fleet of starships was built in orbit of Mars, with which the Emperor's armies set out to reconquer the galaxy for Mankind.
But even before He had begun His campaign to reunite the Earth, the Emperor had used His highly advanced knowledge of genetic engineering and the psychic arts to create the Primarchs, 20 superhuman military commanders possessed of preternatural physical, mental and social attributes who had been created through the fusion of variations of His own genetic material with the power of the Warp. The Emperor had known that the greatest foe of His plans to reunite the human race and bring Order to the galaxy would be the Ruinous Powers of Chaos and their daemonic servants. He had sought to protect His gestating Primarchs from the Dark Gods while they grew in their gestation capsules in the Emperor's secret laboratories deep beneath the Himalazian (Himalayan) Mountains, but the Chaos Gods saw through all His wards and protections. They opened a Warp portal in the Emperor's own laboratory and stole the infant Primarchs away from Terra, scattering them across the galaxy.
Undeterred by this tragedy, the Emperor utilised what remained of the Primarchs' genetic material. He had first crafted the Thunder Warriors to unite Old Earth. He then proceeded in the last days of the Unification Wars to transform ordinary Terran men into a new corps of transhuman warriors who would lead Mankind back out into the stellar void. These men were the Emperor's Space Marines, the Legiones Astartes, superhuman warriors who knew no fear. Thanks to influxes of technology and resources from around the Solar System the Space Marine Legions were soon growing in numbers and capability at an exponential rate. The Space Marine Legions were to be the spearhead of the Emperor's attempt to reunite Mankind -- the Great Crusade's killing edge against which the strength of a foe would be broken and which would topple empires, human or alien, by ripping out their hearts. With Mars now part of the Imperium, the Great Crusade began in earnest and the rest of the Solar System was the first region of space to be conquered by the Emperor and His newly rearmed and re-equipped Space Marine Legions. The Legiones Astartes secured the scientific research installations and spacedocks of Luna which had once been controlled by the Selenar gene-wrights. Broken and humbled, the enslaved gene-wrights of Luna would help forge the next generation of Space Marines who would carry out Mankind's conquest of the stars. Alien invaders were flushed from the moons of Saturn and Jupiter and their wretched enslaved human inhabitants repatriated to Earth, the once-human creatures of the Neptunian Deeps were exterminated without mercy, and the baleful false-world of Sedna at Sol's edge-light was boiled away to vapour under the guns of the new-forged Imperial war fleet. The next step was beyond.The Great Crusade marked the beginning of the period that would later be known as the Age of the Imperium. This campaign was a mammoth operation on an inconceivable scale and complexity that involved billions of troops and tens of thousands of voidships, and it is perhaps true that only a mind such as the Emperor's could have had a hope of successfully comprehending and executing it. In order to manifest this conquest the Imperium's forces were divided up into an expanding and frequently reconfigured series of Imperial Expeditionary Fleets -- semi-autonomous battle groups assigned to voyage the stars and make war in the Emperor's name. they were composed in chief of a bewildering array of voidships great and small. The paths of these fleets weTe dictated both in general by the Emperor and his War Council, but also by the will of their commanders who were entrusted to seek out the enslaved and destroy the dangerous alien under their own sway.
During the 200 standard years that the Great Crusade spread the Imperium across the galaxy, the Emperor slowly made contact once more with all of His scattered gene-sons, the Primarchs, and to each He gave command of the Space Marine Legion that had been created from that Primarch's specific genome. Horus was the first of the Primarchs to be rediscovered, on the barren Mining World of Cthonia only a few light years from Terra itself. Horus campaigned with the Emperor for 30 Terran years before the next of the Primarchs was recovered, and during that time the two developed a special bond, the deep affection between a father and His favoured son. The Astartes Legions, combined with the might of the Imperial Army, reunited thousands upon thousands of human-settled worlds into one interstellar civilisation, in the name, and under the rule of, the Emperor and the Imperium of Man. But the Great Crusade was ended abruptly by the corruption and treachery of the Imperial High Senate, which had become the overall military commander of the Great Crusade in its last days. This was an honour that had been granted to them by the Emperor, who had sought to leave the campaign so that He might return to Terra and oversee a highly-secret project intended to open up the Eldar Webway to human use.
While Horus and many of the other Primarchs deeply resented the Emperor's absence from the Great Crusade and were even more disturbed by His attempts to replace the direct rule of the Imperium by the Emperor and His Primarchs with a bureaucracy of normal humans, they all remained loyal to their father. Having established the Council of Terra, the new governing body of the Imperium to carry out the day-to-day work of ruling tens of thousands of worlds and trillions of human beings, the Emperor took refuge in his vast laboratories and workshops beneath the Imperial Palace. He began work in earnest on his new project to secretly extend the Eldar Webway to Terra, an enterprise that the Emperor hoped would be His greatest gift to Mankind. While the Emperor was locked away in His subterranean factories, trouble was brewing. The formation of the Council of Terra proved to be a contentious decision with the distant Primarchs, who were appalled when news of the formation of the Council finally reached them on the frontiers of the Great Crusade. Some of the Primarchs took great exception to being ruled by those mortals that they deemed less worthy and capable of such an honour than themselves. The less stable Primarchs felt that this was a betrayal of all they had fought and won in the Emperor's name and that their victories now counted for nothing. They, and many of their Astartes, felt that it was they who had suffered and sacrificed the most to build the Imperium and thus it was they who should have the greatest say in how it was ruled, not a council composed of effete Terran nobles and faceless mortal bureaucrats.
After the traitorous Senate had attempted to murder the Emperor and sealing him inside his own laboratories. Their next step was to try to remove the Adeptus Astartes Legions completely from the Imperium, climaxing in the civil war known as "Horus Heresy". After Horus and the other 17 Primarchs had learned what had transpired, they immediately set out to conquer Holy Terra. The planet was now besieged by all 18 Legio Astartes as well as a full third of the Imperial Army. After months of constant siege, the walls protecting the Imperial Palace broke and the blood lusting sons of the Emperor stormed the breach and in a mere hour had removed the Senate from power, by destroying every last member.
Following years of rebuilding, the Primarchs finally learned of the reason why their father had almost been murdered and was only now returning from his coma. The human-centric view that had lead to the Great Crusade that lasted 200 years, was to be abolished instead the Emperor had wanted to integrate all Xenos that were willing into their Empire. Seeing the error of trusting mere humans to control his Imperium, the Emperor re-elected his Sons for this task and trusted them to get the job done.