The Emperor of Mankind is the sovereign of the Imperium of Man, and Father, Guardian, and God of the human race. He has sat immobile within the Golden Throne of Terra for ten thousand years. Although once a living man, his shattered body can no longer support life, and remains intact only by a combination of ancient technology and the sheer force of his will, itself sustained by the soul-sacrifice of countless millions of psykers.
His will is omnipotent, extending across the million worlds that comprise his Imperium. For ten thousand years the Master of Mankind has served humanity, simultaneously carrying out a multitude of tasks vital to its survival. All at once He guides his race through the Emperor's Tarot, soul-binds psykers, holds audiences with his most important servants and directs the Astronomican beacon that guides space vessels through the Warp. His immense psychic powers constantly keep the Chaos Gods in the Warp at bay, preventing their intrusion into the material universe and protecting his people throughout the galaxy.
The Emperor's role as guardian of mankind seems to have been predestined for him. Without the Emperor there would be no Imperium, little space travel, and no protection from the multitude of threats facing mankind. The Emperor knows that to protect his race he must survive as long as necessary for the emerging race of psychic humans to evolve sufficiently.
In the millennia since his ascension to the Golden Throne, the Emperor has become a god to his people, the worship of Him uniting humanity throughout his Imperium, superstition and dogma replacing his doctrine of Imperial Truth.