Emperor of Mankind

"The Emperor Protects..." — The Lectitio Divinitatus
The Emperor of Mankind, often referred to by His faithful as the "God-Emperor," the "Master of Mankind," or simply "the Emperor," is the immortal Perpetual and psyker who serves as the reigning monarch of the Imperium of Man, and is described by the Imperial Ecclesiarchy and the Imperial Cult as the Father, Guardian and God of Humanity.
The Chaos Gods and the Daemons of the Warp refer to Him as "the Anathema" for He is the greatest embodiment of universal order in the galaxy today and the most potent foe of Chaos in existence. He was, and remains, the most powerful Human psyker to have ever been born.
He has sat immobile, His body slowly crumbling, within the Golden Throne of Terra for over 10,000 standard years. Although once a living man, His shattered, decaying body can no longer support life, and it is kept intact only by the cybernetic mechanisms of the Golden Throne and a potent mind itself sustained by the daily sacrifice of thousands of lives.
The Emperor chose to sacrifice His immortal life at the end of the Horus Heresy in the service and protection of Mankind. To Humanity's countless trillions across the galaxy-spanning Imperium, He is nothing less than God. Through His Imperium, Humanity is united and remains one of the most powerful intelligent species in the Milky Way Galaxy as well as its most dominant in terms of both population and territory held.
United under one government, Mankind is able to survive the myriad deadly threats it faces from aliens, the forces of Chaos and the Traitors, Heretics and mutants that lie within the Imperium's boundaries.
The Imperium's rule, carried on in the Emperor's name since the end of the Horus Heresy by the High Lords of Terra and a multitude of Imperial organisations, has been long, oppressive and necessarily harsh.
It has also resulted in technological and cultural stagnation, and a regression into tyranny, superstition and religious obfuscation and intolerance that would have horrified the Emperor.