See Imperium of Man for more information.
More then 25 Millennia ago, humanity first looked to the stars and began the great exploration of nearby space. Limited at first to sublight travel, colonisation and conquest proceeded at a snail's pace. Generations could live and die on the great interstellar voyages without ever seeing their destinations. Colonies were forced into independence; aid took centuries to arrive, if it came at all.
With the discovery of the warp drive came the great expansion. Mankind was freed from the need to spend generations travelling between the stars. The subsequent discovery and refinement of the navigator gene, which allowed pilots to make longer and more accurate warp jumps, merely confirmed the trend. Within centuries, rather than the millennia, humanity burst from its confining systems and spread out into the galaxy.
As humanity moved into the galaxy, the Dark Age of Technology brought seemingly unstoppable progress. Humanity's machines achieved incredible levels of sophistication. There seemed nothing that Man could not do, even as humanity began to splinter into smaller groups. Some were cut off in remote parts of the galaxy; others, deliberately isolated themselves, under their power and knowledge to make war on their enemies, human and alien alike.
It was at this time that the “psykers” - humans capable of using powers such as telekinesis and telepathy – first arose in the midst of humanity. The trigger for this emergence has never been determined, but psykers were recorded on almost every planet known to man. Untrained and unprotected, many went mad or fell prey to non-psychic man; uncounted millions were burnt as witches, destroyed by ignorance. More fell to creatures – Daemons – from warp space, beings which fed upon their minds and bodies. Civilisation crumbled as ignorance and madness replaced enlightenment and technology. The Dark Age of Technology ended; the Age of Strife was born.
For more than five thousand years warfare tore humanity apart. Alien and human, all enemies were as one. The loose confederation of human space was shattered by local wars on every scale: nation battled nation, planet fought planet, system laid waste to system. Outspacers and aliens plundered the ruined worlds. Abandoned or cut off by warp storms, human colonies fell victim to local conditions. Terraformed worlds reverted to their natural condition and as they did so, humanity suffered. Through chance and depleted gene-pools, some colonies underwent a forced evolution, producing new successor species: the stocky Squats, the powerful Ogryns and the Beastmen. Other colonies simply reverted to savagery under the combined pressures of alien raids, ecological disaster, psychic madness and technological failure.
Only the worlds where psykers were rigourously repressed survived intact. The retrenchment of Man was almost total.
At last, as so often happens, a single man, a conqueror, emerged from the darkness. The Age of Strife and its bloody wars were brought to an end. The Imperium of Man was founded as human systems were reconquered and returned to the mainstream of humanity of their own accord. The Emperor reunited humanity under his banner. For ten thousand years this immortal being has ruled the Imperium.
His rule is harsh and uncompromising, but it is only under his guidance that humanity has survived. The spectre of a new and more terrible Age of Strife is ever present; the threat from the enemy within is matched only by the aggression of the enemy without. The Emperor stands between humanity and extinction.