Over ten thousand years ago the Great Emperor of Mankind ascended to the Golden Throne of Earth. Of the wars he waged to get there, of the countless agonies of battling worlds, there is no record. Only the Emperor remembers - if indeed even that strange and ancient creature can recall those distant times.
The ascent of the Emperor marked the end of a long era of human history, an age typified by inter-human warfare and a gradual decline of the accumulated knowledge of millennia. This was the Age of Strife. The high point of scientific achievement had occurred thousands of years before, in the yet older age known now as the Dark Age of Technology. Through the Dark Age of Technology and the Age of Strife, mankind has come to the present age - that of the Imperium.
In many respects it is a time of superstition, in which a great and unfathomable technology has been enslaved to the forces of mysticism and madness. To the ordinary humans of these times, the peasants in the fields and the warriors amongst the stars, scientific thought represents an abhorrent perversity; a corruption of honour and religious virtue. Even to those few humans who deal with the material of technology, the science and the magic have become largely inseparable; the warp engine must have runes upon its side, the laser gun requires the blessing of the Gods of Battle.
This is a universe in which the gods, mysticism and magic are real. For this is a time of great change within humanity itself, a time when more and more humans are developing powers and a vision far beyond those of their ancestors. Humans developing these abilities are known as psychics, or psykers, and by many less flattering names - the most common being that of witch. Psykers are men and women who have the ability to transcend the normal laws of space, placing them in touch with great universal forces which lie far beyond the understanding of their kin. Their gifts defy explanation; telepathy, telekinesis, illusion, and countless others. Many ordinary humans attribute these powers to a divine origin. Some psykers claim their powers enable them to tap forces which he under the control of entities existing outside of the real universe - beyond normal time and space. Others seek a rational explanation for these phenomena - but this is not a rational age and they are few and their voice is weak.
From Warhammer 40,000: Rogue Trader