Timeline

This article briefly summarizes the before, during and after the events of Sins of the Father from the Unification Wars up to the 41st Millennium. All dates are given in the format 999.M41 which should be read as the year 40,999 AD in the Gregorian Calendar. Knowledge of the long ages of human history has been wreathed in shadow, buried beneath the aeons or simply lost over the expanse of time. Some historitors continue to seek knowledge, but their work can be compared to holding a candle against the dark abyss. The historitors of the Adeptus Administratum seek, gather and dutifully collect the history of each planet within the Imperium.

Once, perhaps, this was done with purpose. Now, however, it is most often an indiscriminate compilation of data, a process where scribes unthinkingly copy content onto scrolls, preserving complete records in constant fear of missing a single inconsequential letter. The ability, or even desire, to translate what these facts mean in any larger sense is almost entirely absent, found only in a few individuals who are eyed suspiciously, or perhaps punished for their enthusiasm.

The largest and most complete collection of such records exists beneath the surface of Terra, where the colossal under-halls stretch out of sight -- endless vistas of datascrolls tower upwards like mountains.

With flickering candlelight provided by floating Servo-skulls, processions of curators, scriveners, and ciphers wind through the valleys, attempting to illuminate that which was. Over the years, old truths have been lost to the "adjustments" and redactions made by revisionists of the Adeptus Ministorum, the Inquisition and even the High Lords of Terra themselves.

It is known that both Imperiums, Primaris and Secundus, record their histories differently. The History of Primaris is extremely embellished and buried under superstition whilst Secundus' tends to be more accurate albeit just as embellished. This article will treat both Imperiums as one singular entity for sake of ease.

## Pre-History

Just as the stars gave birth to the mysterious Star Gods so the planets of the newborn galaxy eventually gave birth to lifeforms composed of matter which began the long evolutionary climb to self-awareness. The first sentient beings of the Milky Way Galaxy known to have developed a civilization technologically advanced enough to cross the stars was a reptilian race of beings called the Old Ones by the Eldar, who knew them best. They possessed a slow, cold-blooded, but still deep wisdom, having long studied the stars and raised astronomy and physics to such a level that their science and technology appear to humanity like an arcane art. Their understanding of the workings of the universe were such that they could manipulate alternate dimensions and undertake great works of psychic engineering. Their science allowed them to cross the vast gulfs of space with only a single step through the myriad Warp Gates they built to connect the worlds of the galaxy in a vast network much like the Eldar Webway of today, though on a much larger scale. The Old Ones spread their spawn to many places in the galaxy, but they also knew that all life was precious. Where they passed, they seeded new intelligent species and reshaped thousands of worlds to make them their own according to their predetermined environmental and geographic criteria. It is believed by some in the Adeptus Mechanicus that even Terra felt the Old Ones' touch long before humanity's rise to self-awareness, though this notion is considered heretical at best by the Ecclesiarchy, as the Imperial Creed teaches that Mankind was made in the image of the God-Emperor before his spirit was incarnated in physical flesh millennia ago.

The Old Ones' civilisation reached its height in excess of 60 million years ago. The Old Ones were responsible for the creation or genetic advancement of most of the currently active intelligent species of the galaxy, including the Eldar, the Krork (the Orks' precursors), the Slann and the Jokaero, though it is unknown if they played any role in the evolution of humanity. The Old Ones were potent psychics who routinely used the powers of the Warp for a wide variety of technological applications. The Old Ones constructed a system of instantaneous faster-than-light portals through Warpspace that were ultimately adapted to create the Eldar's Webway (and was its more advanced precursor). These portals connected all of the Old Ones' colony worlds across vast swathes of interstellar space.

Unfortunately, the Old Ones would be destroyed in the destructive War of Heaven by the undying Necrons, who had been ascended from a dying race to an immortal one by the trickery of the Star Gods, or the C'Tan as they were called. With the C'tan and the Necrons fighting as one, the Old Ones were now doomed to defeat. Glutted on the life force of the Necrontyr, the empowered C'tan were nigh unstoppable and unleashed forces beyond comprehension. Planets were razed, suns extinguished and whole star systems devoured by black holes called into being by the reality-warping powers of the Star Gods. Necron legions finally breached the Webway and assailed the Old Ones in every corner of the galaxy. They brought under siege the fortresses of the Old Ones' many allies amongst the younger intelligent races of the galaxy, harvesting the life force of the defenders to feed their voracious C'tan masters.

In the closing years of the War in Heaven, one of the primary factors that led to the Necrons' ascendancy was their ability to finally gain access to the Old Ones' Webway. The C'tan known as Nyadra'zath, the Burning One, had long desired to carry his eldritch fires into that space beyond space, and so showed the Necrons how to breach its boundaries. Through a series of living stone portals known as the Dolmen Gates, the Necrons were finally able to turn the Old Ones' greatest weapon against them, vastly accelerating the ultimate end of the War in Heaven.

The portals offered by the Dolmen Gates are neither so stable, nor so controllable as the naturally occurring entrances to the Webway scattered across the galaxy. Indeed, in some curious fashion, the Webway can detect when its environs have been breached by a Dolmen Gate and its arcane mechanisms swiftly attempt to seal off the infected spur from the rest of the Labyrinthine Dimension until the danger to its integrity has passed. Thus, Necrons entering the Webway must reach their intended destination through its shifting extradimensional corridors quickly, lest the network itself bring about their destruction.

The War ultimately ended with the Old Ones' extinction, the Immaterium becoming a hellish dimension, and the Necrons rising up against their C'Tan masters. The Necrons had been vindicated in their pursuit only of science and control over the material realm and certainly took pleasure in seeing the Old Ones' civilisation collapse as a result of their over-indulgence of psychic power and the end of the C'tan's domination over their race. Yet even with the defeat of the Old Ones and the C'tan alike, the Silent King saw that the time of the Necrons in the galaxy was over -- for the moment, at least. They would allow the Enslavers to take what was left of the sentient life in the galaxy and let it become an interstellar wasteland; the psyker swarm would then die away and in time the galaxy would evolve new lifeforms who would be less sophisticated and easier to dominate. In addition, the Necrons understood that the mantle of galactic dominion was soon to pass to the Eldar, one of the psychically-potent races that had fought alongside the Old Ones throughout the War in Heaven and had thus come to hate the Necrons and all their works with the burning passion that is the defining characteristic of that species. The Eldar had survived where the Old Ones had not and the Necrons, weakened by their expenditure of lives and resources in overthrowing the rule of the C'tan, could not stand against them. Yet the Silent King knew that the time of the Eldar would eventually pass, as it must pass for all those beings still cloaked in the flesh. It would take millions of Terran years for the Eldar's power to fade, but what mattered is that the Necrons would be there to take advantage of it.

Thus the Necrons went into hibernation, one they have yet to awake from. Being ageless and undying, the Necrons are in no hurry to wake up. Time is, and always will be, on their side...

## Rise of Mankind

### The Age of Terra and the Stellar Exodus

For millions of standard years after the Necrons and their C'tan masters went into hibernation on their Tomb Worlds, the devastated sentient populations of the galaxy slowly recovered from their "red harvests." In time, the Eldar emerged as the most dominant civilisation in the galaxy, with the core of their star-spanning empire located in the region of what would later become the Eye of Terror Warp rift. But on the planet called Earth and later Terra by its inhabitants, a humanoid species known as Mankind was rising to prominence over 40,000 standard years ago. During the Age of Terra, or the Age of Progress as it is sometimes called by Imperial historians, the human race advanced beyond its ancient pre-industrial past to obtain spacefaring capability and began to slowly settle the habitable worlds in its own solar system and in the star systems near its homeworld using massive sublight starships.

The Stellar Exodus which occurred during the Age of Terra is a poorly-understood period of human history which is generally accepted to cover the majority of Mankind's initial forays into interstellar space and the genesis of most of the oldest human colonies, beginning at some unknown point in the mid-to-late 3rd Millennium. This period is generally understood to lead into the Dark Age of Technology sometime around the 15th Millennium. During much of the Stellar Exodus, humanity lacked any knowledge of the existence of the Warp or Warp drive and so was forced to travel between the stars in great sublight generation ships or using cryogenic hibernation.

### Dark Age of Technology (M15-M25)

Much of the era of human history known to Imperial historians as the Dark Age of Technology that lasted between approximately the 15th Millennium and the 25th Millennium is mysterious. It was in this time that the psychic mutants called Navigators were first born, and combined with the invention of the Warp-Drive, allowed humanity to travel between worlds faster than the speed of light by using the dangerous hyperdimensional, psychically-reactive medium of the Immaterium or Warp. The existence of rapid interstellar travel allowed human colony worlds to interact on a normal timescale for the first time and interstellar commerce and communication became possible, allowing like-minded human worlds to join into political and economic combines for their mutual benefit. Eventually, at some point during this era, all of humanity was united beneath some form of federated interstellar government. What is also known (largely from the works of Keeper Cripias) is that a group known as the Golden Men relied on the works of the Stone Men in order to create a fantastically prosperous interstellar society, but one devoid (by later Imperial standards) of spirituality or piety, focused instead upon the advancement of science and technology. The Stone Men are known to have created a third group, the Iron Men, postulated to be some form of artificially intelligent robots, in order to assist them in their labors and carry out military duties. The Iron Men became uncontrolled and rebelled against their human creators, and a cataclysmic conflict broke out, resulting in a partial Malthusian Catastrophe across much of the settled galaxy in which there were no longer enough resources to support the vast populations of human beings. Even worse, following the destruction caused by the war against the Iron Men, human psykers began to appear in large numbers across many human colony worlds, one of the harbingers of the final decline of the Eldar empire which controlled even more of the galaxy than mankind did in these years. The result was humanity's first introduction to the daemonic horrors of the Warp and the ultimate collapse of human interstellar civilization during what became known as the Age of Strife.

### The Age of Strife (M25-M30)

During the so-called "Dark Age of Technology," humanity had reached its scientific and technological peak. The "Standard Template Construct", or STC, a computer database of schematics for all known advanced human technologies, had been perfected by human scientists and engineers and allowed an unprecedented expansion of humanity throughout the galaxy. One of the reasons humanity was so successful at conquering a large part of the galaxy was the development of the artificially-intelligent constructs now only known in Imperial legend as the "Iron Men". These powerful and fully intelligent combat robots won many wars for humanity, but for some reason turned against their masters at the end of the Dark Age of Technology. The war was eventually won by humanity, but at a great cost. The damage was catastrophic and had shattered much of humanity's power. Unfortunately, this was only the beginning of mankind's misfortunes.

As humanity became widely dispersed across the galaxy during the Age of Technology, the ancient Eldar empire began its decline; the great success of the advanced Eldar race had led to decadence and hedonism on a grand scale. Within the immaterial, psychic universe of the Warp, the spiritual corruption of the Eldar civilization was reflected in the forming of a new Chaos God, Slaanesh, which in turn caused massive disturbances in the Warp; parts of the galaxy became isolated by these Warp Storms, making Warp travel and telepathic interstellar communication increasingly impossible as the years passed, cutting off many human colony worlds from one another, including those of the Solar System.

Towards the end of the Age of Technology psykers first appeared among humanity. While persecuted on many backwards, regressive human worlds as witches, in enlightened and progressive societies these psykers were at first protected and accepted. The initial intolerance for psykers would later seem prescient, as many human worlds fell to the dominance of daemons and other Warp creatures using possessed psykers as gateways into the physical world. Only worlds which had rigorously suppressed psykers survived the Age of Strife.

The Age of Strife followed the Age of Technology, as human civilisation collapsed in widespread insanity, daemonic possession, anarchy and inter-human civil war. Terrible weapons of the golden age of technology were unleashed, devastating many human colonies and turning once-verdant worlds such as Baal Secundus into irradiated desert planets. Many isolated and vulnerable human-colonized worlds also became prey to hostile alien races, such as the Orks.

In a relatively short span of time, the once galaxy-spanning human civilization was brought to its knees, and was forced to endure nearly five millennia of anarchy, terror, war, genocide and slavery. Other than tales of great suffering, little information concerning these long years survived this dark time to be known to the men and women of the Imperium.

By the late 29th Millennium, it had been nearly three hundred Terran centuries since Humanity first left the polluted, used-up carcass of what was once known as Earth for the unknown bounties and perils of the stars beyond the world of its birth. In those forgotten early days of the Age of Technology it is now unknown what enemies and trials Humanity had to overcome but overcome them we did, and in doing so Human colonies spread across our galaxy so that Mankind was scattered across a million worlds and found treasures and horrors uncounted.

In the heights of those times, lost now to all but supposition and myth, Humanity made themselves as unto gods -- their sciences and arts able to remake planets in the image of remembered OLd Earth, harness the power of the stars, and fashion servants from clay and iron and give them counterfeit-life to do their bidding. Thus was the Dark Age of Technology forged and for millennia it reigned, until almost at a stroke the dominion of Humanity fell and was undone in a maelstrom of brutality and ignorance and untold billions died, their screams unanswered and unheard.

During the turbulent era known later as the Age of Strife or by some as "Old Night," the Sol System and Old Earth, or Terra as it is now known, and the nearby star systems that had been colonised by Humanity during the Dark Age of Technology were effectively cut off from interstellar travel or communication with each other. This was due to the massive Warp Storms that swept the galaxy as the Immaterium was roiled by the millennia-long gestation of the Chaos God Slaanesh and the turbulence that marked the decay of the Aeldari Empire before the Fall.

During this dark time, Terra sometimes held sway over the Sol System, while at other times the rulers of Mars or Luna were dominant. The different worlds found themselves constantly at war. During this long, 5,000-year-period of anarchy, fear and violence, Old Earth's once unified planetary government had completely broken down and been divided into dozens of warring, backwards states of so-called "techno-barbarians."

Continuous warfare raged across the surface of Terra for 2500 standard years, beginning in the middle centuries of the 27th Millennium. Across the galaxy things were little better, as the Human colony worlds, bereft of central authority or protection, and with the web of support and supply that had maintained the threads of civilisation cut, fell into anarchy and war.

Human civilisation was torn apart as local factions and pocket empires fought for control in the shattered realm while others, utterly cut off from aid, succumbed to the horrors of planet-wide famine, insanity and worse -- the perils of the Warp. The existence of Warp creatures and the dangers they posed to the Human mind were then barely understood. On worlds with large concentrations of emergent psykers the entities from beyond were able to breach the barrier between the Immaterium and corporeal reality and it cannot be known or guessed how many worlds were ravaged or swallowed whole by their incursions.

As Human civilisation fragmented, hundreds of alien species and enemies unknown seized their chance for revenge on Humanity for its past conquests, or to plunder unprotected worlds and enslave their populations. Hundreds of Human-inhabited star systems were sacked and ravaged, their peoples slaughtered, mutilated or stolen. Those that survived the alien onslaught rapidly reverted to barbarism, stripped of civilisation, knowledge and culture in the raw battle to simply endure.

### Fall of the Aeldari (M25-M30)

Before the Fall of their potent interstellar empire, the Eldar were a technologically and psychically advanced humanoid species, generally considered the most powerful intelligent race in the entire galaxy. Their technology had advanced so far that little or no labour was required by individual Eldar to provide the daily necessities of life. As a result of the sheer boredom and ennui that resulted, at some point during the 25th Millennium, groups of Eldar began forming what were known as Pleasure Cults dedicated to the pursuit of every pleasure and sensation that life had to offer their kind.

Despite the prediction of the reclusive Eldar Seers that warned of impending doom if the Eldar did not change their hedonistic ways, government within the Eldar empire soon collapsed and the moral degeneration of their homeworlds and various colony worlds continued unimpeded. As the pursuit of ever more extreme experiences reached its height, death reigned in the streets of Eldar cities, hunter and hunted each being part of a twisted ritual of destruction that consumed millions in orgies of savage bloodlust and excess. Some Eldar were able to see that their now-corrupt society was destroying itself, and fled in disgust; these refugees would settle in the distant, rural colonies of their empire, and would later be known as the Exodites or take to the massive, world-sized Eldar Craftworlds and sail amidst the stars as merchants and explorers, far from the scenes of hedonistic excess that were destroying the heart of the Eldar empire.

Upon dying, the soul of all deceased, sentient beings traverse the bounds of the physical realm and go to rest within the hyperdimensional realm of psychically-reactive energy that undergirds all of reality known as the Immaterium or Warp. As more and more corrupt Eldar died, their souls began to coalesce into a larger entity within the Warp, a living representation of the hedonistic corruption that had taken their lives. This growing presence in the Warp caused massive Warp Storms to aggregate across the galaxy, eventually making interstellar travel and communications impossible for the colonies of humanity, bringing on Makind's Age of Strife. This collection of corrupt Eldar souls gained sentience sometime in the late 30th Millennium, creating the powerful new Warp entity known as the Chaos God Slaanesh, the Prince of Pleasure, called "She Who Thirsts" by the Eldar. When Slaanesh came to be, an ethereal explosion occured that spanned the extent of Warpspace within the Milky Way Galaxy, with the epicentre being located within the Eldar homeworlds. All Eldar caught in the immense psychic blast were instantly killed, their souls consumed by Slaanesh. Most of the remaining Eldar gods who existed within the Warp were destroyed by Slaanesh. Kaela Mensha Khaine, the Eldar God of War, attempted to combat the new Chaos God, but Khaine's form was shattered and exiled to the mortal realm where it came to rest in the Infinity Circuits of the Eldar Craftworlds but could be embodied in the form of animated constructs called "Avatars", which rested in the hearts of the various Craftworlds during times of grave peril. The only other Eldar Gods to survive the Fall were the trickster god Cegorach (also called the Laughing God), who hid himself within the Eldar Webway, and Isha, who was ripped from the jaws of Slaanesh by the Chaos God Nurgle, who now keeps her as a slave.

The Fall utterly destroyed the once vast interstellar Eldar empire, leaving scattered bands of Eldar fighting for survival. Before the Fall, vast spacecraft called Craftworlds were constructed, enabling those wishing to escape the degeneration of the Eldar homeworlds to flee, along with samples of their lost worlds' flora and fauna. When the Fall occurred, the various Craftworlds rode out the ethereal shockwave in the Warp, some being destroyed in the process. The Exodite worlds, far from the epicentre of the terrible catastrophe, were largely untouched by the birth-scream of Slaanesh. In order to prevent the events of the Fall from ever recurring, the Eldar devised the Eldar Paths system, by which every Eldar would follow a strict, almost fanatical life path pursuing a specific activity, such as crafting or war, for a portion of their long lifespans, before switching to another Path when the previous one grew stale. In this way, the Eldar hoped to control the extreme emotional and psychic sensitivity that had left them open to corruption by Chaos in the first place. However, some corrupted Eldar did survive the birth of Slaanesh by taking refuge within the Webway itself, which had long been home to a nearly infinite cavalcade of hyperdimensional sub-realms that were protected from Slaanesh's predations by the psychic wards erected within the Labyrinthine Dimension to keep the dangers of the Warp at bay. These sadistic beings became known as the Dark Eldar, the inhabitants of the Dark City within the Webway called Commorragh, and they later preyed on humans and Eldar alike to find souls they could devour to keep their own lives from being consumed by Slaanesh.

## The Great Crusade (ca. 798.M30-020.M31)

### Wars of Unification

During the Age of Strife, Terra, the birth­world of Humanity, had descended into savagery and horror the equal of any. Little remained of the once sophisticated civilisation of Old Earth's glorious past as the centre of a growing Human interstellar civilisation marked by advanced science, high culture and wondrous technologies.

Techno-barbarian warlords and their warrior hordes continuously fought over the planet, which had become little more than a massive battleground for their wars of attrition. They made use of chemical, biological and even thermonuclear weapons of mass destruction, and slowly transformed the cradle of Mankind into a battered, post-apocalyptic wasteland across most of its scarred surface. Long before left parlous and polluted, generations of war made the planet a wasteland punctuated by fortified holdfasts built amid the wreckage of lost civilisations, and its inhabitants a mass of degenerate, feral nomads.

Insane prophets, cyber-augmented burcher-kings, vivisecror-warlords and religious demagogues led the warring tribes of Ancient Earth, and the world was wracked by never-ending conflict as one tyrant displaced another. Petty empires rose and disintegrated; tribes formed, were destroyed and re-formed as diminishing, brutalised shadows of lost glories and forgotten triumphs. Uncounted billions died unknown and unremembered, while whispered names such as the "Unspeakable King" and the "Seven Neverborn" left legends to terrify generations to come. Anarchy and bloodshed was lord over all.

But not for long.

It was against this backdrop of oppression, violence and casual brutality that the Emperor of Mankind first revealed Himself to the people of Terra beginning in the 29th Millennium as a warlord more powerful than any other. In secret, He had been planning for this moment in history for millennia, ever since the Age of Strife had fractured what remained of the ancient Human federation which had once stretched across the galaxy.

With His massive army of genetically-enhanced warriors who comprised the first units of the Imperial Army and would serve as the prototypes for the later development of the Space Marines, the Emperor began His conquest of Terra with the intent to reunite the warring nations into a unified planetary government. But the Emperor's true power lay in His rationality and foresight, and His mastery of science was as unequalled as His might as a warrior, although few suspected the true range of His powers and abilities.

With His Legions of Thunder Warriors, the Old 100, the Legio Custodes and the Proto-Legions of the Astartes, the Emperor steamrolled all opposition albeit at heavy cost, for the Unification Wars was little more than a meat grinder. The Emperor accepted the deaths of the many innocents that resulted from the conquest of Terra with great remorse in order to achieve the greater good of unifying Humanity and protecting it from the manifest predations of the Warp and predatory xenos of the galaxy.

Unimaginably large, full-scale battles during the Unity campaign would last for solar weeks on end, with body counts in the millions that sundered mountains and split entire continents. Future Imperial scholars would later dismiss these victories as lurid hyperbole, refusing to believe that such clashes of arms could possibly have been fought, but indeed they were. he Imperial historical chronicles tell of the last battle of the Unification Wars, known as the Battle of Mount Ararat, which was fought in the Kingdom of Urartu. During this final battle the remaining Thunder Warriors were slain to a man, sacrificing themselves to win the final greatest victory for the Emperor.

Unfortunately, the truth is far from glorious sacrifice. In actuality the Emperor had the Thunder Warriors culled, slaughtered by the Custodes and the early Ist Legion. The reason for the culling was the Thunder Warrior's instability, both mental and physical, being soley created for war and nothing else they would've posed a threat to the stability of the new Imperium. Therefore, they could not be allowed to survive the conflict they had been created to fight.
Under the Emperor's rulership, Humanity would be restored, intellectually, spiritually and physically to the heights it had once known, and the shackles of darkness cast aside.

The war continued without relent until finally all of Old Earth's techno-barbarian factions, nations and fractured city-states pledged the Emperor fealty and at last Terra was unified for the first time in many millennia. It was not enough merely to secure military victory, however -- the betterment and restoration of Humanity required more. So it was that even before victory on Terra had been achieved the Emperor and those learned souls He had gathered to Him started to experiment with genetics.

This was done first and foremost to stabilise the extant population of Terra and to recreate the race of Mankind as it was before the radiation storms and generations of viral and alchemical weapons had wracked the planet. Secondly, the creation of genetically augmented transhuman fighters that had begun with the Thunder Warriors who had fought in the Emperor's armies continued apace into newer, more powerful creations, ultimately leading to the genesis of what would later become known as the Space Marine Legions.

But in truth, the Emperor's greatest work was nothing less than the purging of Mankind's own genome, and its restoration to a baseline of what could be considered truly "Human" as that term had been defined before the Age of Strife. The galaxy would belong to Humanity, but it would be the same Humanity that the Emperor had loved and served for most of His long life.

Alongside this ascendancy would come a rebirth of civilisation and government on Terra with the Emperor founding an all-embracing civil administration to order and rebuild His world, and concentrate such learning and knowledge that could be salvaged from the dust of ages in houses of learning and education. And the unification of Terra would only be the start.

Once the Unification Wars were complete in the middle centuries of the 30th Millennium, the Emperor forged a new unified planetary government under His leadership. He next journeyed to Mars and met with the Adepts of the Cult Mechanicus. In return for the use of the ancient Mechanicum's vast manufactoria, the deployment of its Titan Legions, Knights, Skitarii Legions and the orbital shipyards to construct the weaponry and starships He would need for the Great Crusade, the Emperor agreed to grant the Mechanicum complete autonomy on Mars and its other Forge Worlds as well as an exemption to the atheism required by the Imperial Truth.

This agreement, known as the Treaty of Mars (the Treaty of Olympus to the Mechanicum), marks the true foundation of the Imperium of Man in the alliance between Terra and Mars.

### Crusade to the Stars

The Great Crusade of the Emperor of Mankind began as the Age of Strife came to an end for humanity. The Warp Storms isolating the human colony worlds for 5,000 Terran years had finally disappeared with the birth of Slaanesh and the Fall of the Eldar in the 30th Millennium, and the Emperor of Mankind, who had united Terra under his rule during the Unification Wars at the end of the Age of Strife, was ready with his genetically-enhanced Space Marines and the Imperial Army to reunite all of humanity under his enlightened rule and the rationalist philosophy of Imperial Truth. Making a pact known as the Treaty of Mars with the Cult Mechanicus of the Machine God on Mars in the late 30th Millennium the Emperor promised to spare their lives (the Emperor despised their use of bionics and rigid adherence to organized religion as being against everything that he stood for) so long as they aided him in his mission to reunite humanity across the galaxy. The Emperor was a man of enormous skill in the practice of science and the development of new technology, and so in him many members of the Cult of the Machine saw the coming of the Omnissiah that had been prophesied by their faith. In the treaty they signed with the Emperor, the Mechanicum of Mars pledged their support to the creation of a new Imperium of Man in which they would serve as the Adeptus Mechanicus, and the symbol of the Imperium changed from the lightning bolt used by the Emperor during the Unification Wars on Terra to the double-headed eagle known as the Aquila, to symbolise the union of the empires of Earth and Mars. With the forge factories and industrial output of Mars, the Emperor was able to refit his armies, and more importantly, he now had the use of the Adeptus Mechanicus' Titan Legions, giant robotic war machines with the potential to dominate the battlefield.

At first, the Imperium's expansion into interstellar space was slow, since the Imperial Army was still small, and more importantly the Emperor's 20 Space Marine Legions were inhibited by the absence of their Primarchs, which made the enhancement time for the creation of a new Space Marine much longer. However, this was to end, as on Bargrund, a planet in a star system not far from Terra, the Emperor for the first time was reunited with one of his missing Primarchs, Kasimiron Argeryn. Having been discovered at an early age, the Emperor took Kasimiron under his wing and taught him all he knew. Kasimiron and the Emperor had a truly unique bond, that of father and son, and many times they saved each others' lives. But in time, the Emperor discovered another of the Space Marine Primarchs, Zakarah Sawul. Although Horus was pleased at the discovery of one of his siblings, he secretly hoped to always be the Emperor's favorite son.