The Imperium of Man has endured a hundred centuries of strife and bloodshed since the undying God-Emperor was sealed into the golden throne on ancient terra and spoke no more. Over centuries the ancient, monolithic institutions which extend his rule across a million worlds have swollen to the size of nations. Each claims to speak with the voice of the Emperor, be it the Adepts of Terra, the Ministorum, the Space Marines, the tech-priests of the Mechanicus, the Inquisition or another of the million lesser agencies of Imperial rule.
To live in the Imperium is to be part of the harshest regime imaginable, for if there is one thing in great supply it is human blood and sweat. Every citizen keeps a zealous watch for the taint of heresy or deviance in his neighbours. Purity and reverence for the God-Emperor is not only expected, it is demanded. Mankind itself is locked in a war of racial survival where armies of millions can be raised and destroyed in a single day. The galaxy is a hostile place teeming with voracious aliens like the brutal, barbaric Orks and the enigmatic Eldar. No technology, no stratagem can keep such foes at bay, flesh and blood, duty and sacrifice alone can keep the Imperium alive. Endless wars, crusades and purges ripple across the stars as mankind fights for its very existence.
To breach the impossible distances between worlds Human ships must brave the terrors of warpspace; an alternate, nightmare dimension, a sea of souls seething with psychic energy. Warpspace is vital to interstellar travel and communication but it is not only riven by warpstorms but the haunt of terrifying entities which hunger for the existence of the living. The most dark and ancient of these horrors possess unimaginable powers, and style themselves as gods of the nether-realm. Their insane worshippers attack the Imperium from without and within, striving to bring about the ruin of civilisation in exchange for the obscene promise of power and riches whispered by malevolent intelligences from beyond.
This is the future of man, a future where superstition and dogma are carried among the benighted stars aboard antiquated warships and salvation is won through suffering. Not a good place to live, but a good place to die.