The Imperium of Man in brief

The Imperium of Man is the galactic empire under which the majority of humanity is united. The founder and ruler of the Imperium is the god-like Emperor, the most powerful human psychic to date. Founding the Imperium ten thousand years ago, he continues, at least nominally, to lead it.
The Imperium is the largest and most powerful political entity in the galaxy, consisting of at least a million worlds, which are dispersed across most of the Milky Way galaxy. Consequently, an Imperial planet might be separated from its closest neighbor by hundreds or thousands of light years. As a stellar empire, the size of the Imperium can not be measured in terms of continuous territory, but only in the number of planetary systems in its control.
Several aliens and forces (the forces of Chaos, Tyranids, Eldar, Dark Eldar, Orks, Tau, Necrons) challenge the supremacy of the Imperium. From within, the Imperium is threatened more insidiously by rebellion, mutation, dangerous psykers, and subversive cults.
Without the protection of the Imperium, mankind would fall prey to the countless perils that threaten it.

The Imperium was founded by the Emperor, also called “The Immortal Emperor”, “God-Emperor”, and the “Master of Mankind”, at the end of the Age of Strife – a very long period of anarchy, war, and destructive regression which brought humanity to the brink of destruction and reversed the technological progression made in the Dark Age of Technology.
As the Warp storms of the Age of Strife subsided, after having secured the scientific posts and spacedocks of Luna, along with the factories and scientists on Mars, the Emperor began to unite mankind under his rule. A vast navy was built, with which his armies undertook the Great Crusade, which lasted for about two centuries.
During this two hundred year long military campaign, the Emperor employed his most potent military units, the Space Marine Legions, lead by their leaders, the Primarchs. These, combined with the might of the Imperial Army (which was later separated into the Imperial Guard and the Imperial Navy), brought thousands of human worlds together under the Imperium. The Great Crusade ended with the corruption and treachery of the Primarch Horus, the instigator of Horus Heresy rebellion.
The rebellion was fought across the galaxy. Horus seeking to achieving a swift and decisive victory, lead most of the traitor forces directly for Terra. In the final decisive battle between the Emperor and the Arch-Traitor Horus, Horus was slain, leading to the break up of the rebellion. However the Master of Mankind was himself mortally wounded. According to his instructions he was placed on the life-preserving Golden Throne, where, for nearly ten thousand years, he has remained. Though physically a carcass incapable of movement or communication, his will, omnipotent, extends across the million worlds of the Imperium, beams the psychic energy of the Astronomican, soul-binds psychic humans and struggles against the daemons of the Warp. He endures by his undying will, and by the sustenance that only the souls of sacrificial psykers can provide. By the numberless masses of humanity he is worshipped as a god.

The Imperium of Man in brief

Illumination PatrickSawford