The Emperor of Mankind (Also called God-Emperor, Omnissiah and The Anathema) is a 48,000 year old psyker and man of peak physicality who
rules on the Golden Throne forever and ever is decrepit and being kept barely alive for the sake of warp travel and keeping Chaos at bay HERESY! Please report to your Imperial Guard Unit's assigned Commissar for execution.
Born in Central Anatolia, as the result of a shamanistic gathering wishing to ward off the influence of Chaos in the 8th millennium BC, the Emperor of Mankind (real name: Unknown) is one of the most influential men in history. While tracing his history back through imperial records it becomes hard to quantify the years before the Age of Strife. At that time he was a warlord who conquered Earth with an iron fist. In so doing he spread reason, science and atheism as he went (burning churches and houses of worship wholesale), since he knew that the prayers of the religious were sustenance for the Chaos Gods.
Once Earth was conquered he set out to guide humanity and defend it against the onset of Chaos in a campaign known as The Great Crusade (no, not that one). His two major gifts were the Astronomican, a psychic beacon that allows for safe travel through the Warp, and the project that would eventually give birth to the Adeptus Astartes (Space Marines to the non-Gothic speakers, Angels of Death to everyone). After being betrayed by his first child he is hastily placed on the life-support-enabled Golden Throne to, in turn, act as a sort of permanent life support for the Imperium of Man. The Imperium continues to rely on him to protect against Chaos and power the Astronomican.
In this near-death state he constantly requires sustenance of psychic energy leading to one thousand psykers a day being sacrificed to keep his barely functioning brain from going brain dead and the Imperium from being obliterated. He is worshiped as god for the things he has done while most of the knowledge he has imparted on mankind has been replaced with rites and rituals.
 The Irony
The Emperor revealed himself while bringing the blessings of science, enlightenment, reason, atheism, unity, protection, technology and morality to mankind. After he is put on the Golden Throne he is declared as god, has his enlightenment reduced back down to rites and rituals and the Imperium of Man decay into the ultimate fascist wet-dream.
"Did not go as planned" would be an understatement.
 See Also
- An abridged testament to his word
- A less easy testament to his word
- And the funny one
- Fun:Didit generator
- ↑ At least in universe. Today he would be about 10,000.
- ↑ Did I mention he actually looks middle-eastern unlike some other fictional middle-eastern deity?
- ↑ There are four of them. Khorne, the Blood God; Tzeentch, the God of Changing Stuff (really); Nurgle, the God of Decay and Waste; Slaanesh, the God(dess) of Lust, Pleasure and Pain.
- ↑ More complicated explanation: Twenty primarchs were created based on the Emperor to serve as the genetic base for the Space Marines, the first of which was Horus. His heresy crusade (aided by 8 other primarchs, their Space Marine legions and the power of the Chaos Gods) against the Emperor (who had 9 primarchs and armies of his own) caused huge casualties for both sides, leaving Horus dead and the Emperor gravely wounded.