The Master of Mankind, the Emperor of the Imperium, has sat upon the Golden Throne of Earth for ten millennia. His body is kept alive by means of ancient technology and sheer effort of will, for the Emperor is the greatest psyker of all, an almost bottomless repository of psychic energy. He is no ordinary man - in many respects he is a god, and is worshipped as such by untold billions. Ultimately, the Emperor has absolute power within the Imperium.
The Emperor of the Imperium, Master of Mankind, Lord of Humanity and God of the human race, has ruled his vast spatial realm for longer than any living man can remember. Countless millennia ago he was born to mortal parents, growing into manhood little realizing the fate awaiting him. As a youth he began to manifest strange powers, powers which intensified and multiplied as he grew older. Not least amongst these powers was that of longevity - a virtual immortality that gave him time to develop his abilities fully. For long ages he lived secretly amongst mankind, as empires grew and fell, and mankind discovered how to control and exploit the Earth. As his powers evolved he learned of the dangers beyond his own world, of the psychically attuned creatures that roamed the voids in between space, hungering and clawing for the life-stuff of living creatures. For countless ages he hid within humanity, nurturing his powers and waiting. At last, over ten thousand years ago he began his struggle, for he knew that humanity was on the verge of a revolution, a genetic revolution which would create a new psychically aware race, a race of which he was the first and most powerful. Without his guidance he realized the emerging race of psychics would fall prey to the dangers he had already faced, the perils of entities that fed upon psychic energy, or who used that energy for their own horrific purposes. So, the Emperor emerged from long hiding, creating the Age of the Imperium over ten millennia ago in a series of wars now remembered by none save their victor. His rule has been a long and harsh one, for there is much at stake - the life of humanity itself. The strain of his constant vigilance has taken a heavy toll upon the man that was once human, for now his body can no longer support life, and his shattered carcass remains intact only because it is held by a spirit itself sustained by the strangest of machinery - ancient artifacts constructed by the Emperor in an elder age.
It is ironic that this creature, whose will extends to over a million worlds, is now unable to leave the life-giving machinery of his imperial throne, unable to so much as lift a shriveled finger or twitch a shrunken eye. The living carcass of the Emperor is immobile, held fast within the bio-machine that sustains his spirit. The mass of this machine is contained within the imperial palace; room upon room of twisted technology, pulsing with a life and will of its own - living, breathing, reproducing and writhing like a giant, mindless organism. Held within this perversion of science lies the Emperor himself, or rather what now remains of his carcass, the seat of his omnipotent will.
The Emperor understands the dangers that face his race, and has assumed the role which seems preordained for him, that of its guardian. Perhaps he is a freak, or perhaps nature created him as the protector of her metamorphosis. Either way, the Emperor is now the custodian of his race, and he alone bears the knowledge of its fate. To this end the Emperor maintains strict control over the development of humanity and contributes directly to its survival by utilizing his powers. He plays a vital role in space travel within the Imperium. In order to steer a craft over great distances, a human navigator uses a mental homing signal, a sort of psychic beacon to guide him through warp space. To provide a mental signal throughout human controlled space would not be possible to any ordinary psyker. However, the Emperor is no ordinary psyker - his powers go beyond those of mortals. Even so, the strain of transmitting a continuous signal would prove far too strenuous, and he merely concentrates his powers on directing a signal created by others. These are the imperial servants known as the Adeptus Astronomica, psykers whose bodies and souls are leeched of energy. This energy is projected by the mind of the Emperor in the form of the psychic beacon known as the Astronomican. The sheer quantity of mental energy is vast, and only the mind of the Emperor is sufficient to handle so much raw power.
The fate of the Adeptus Astronomica is a sad one, for their efforts soon reduce them to empty husks of bone and dry flesh. Many die every day. They are not the only psykers who are asked to make the ultimate sacrifice, for the Emperor cannot eat as men eat, or drink fluids or breathe air. His life has passed beyond a point where such things can sustain him. For the Emperor the only viable sustenance is human life force - soul - and he has a great and insatiable appetite. Nor will just any human suffice for this purpose, for the soul-donor must be a very special person in their own right, someone with psychic powers. The Inquisition scours the Imperium in a tireless search for emergent psykers, individuals too vulnerable to be left alone. Some of these men and women will be recruited into the Adeptus Terra (especially the Adeptus Astronomica and Adeptus Astra Telepathica) but many more will serve their Emperor in a more gruesome way. Given up to the weird machinery that surrounds the Master of Mankind, their souls will be gradually leeched from their bodies to feed the Emperor's spirit. Hundreds must die in this way every day if the Emperor, the Imperium and humanity are to survive.
It would be simple to think of the Emperor as an evil corruption of nature. Yet, as the Adeptus Terra teach, the sorrow and slaughter that feeds his divine corpse is a trifling price to pay for the survival of the race. Without the Emperor there would be little space travel and no protection in a hostile universe. Left uncontrolled, the emerging race of psychic humans would become the unwitting vehicle of humanity's destruction. For there are many foil aliens which not only feed upon the life-force of other races, but which use that life-force as a means of opening portals in warp space, infiltrating populated planets via the poorly protected minds of inexperienced psykers. The master of Mankind knows that to protect his race he must survive, must live forever if necessary, or until such time as psychic humans have evolved sufficient strength to withstand the dangers they face. If thousands much endure pain and death for his sake, how considerable must be the agony of a creature whose body is all but destroyed, whose mind is encased inside a rotting shell and whose every thought is enslaved to the task of serving his race.
From Warhammer 40,000: Rogue Trader