Just for fun I copied this from Realms of Chaos, Slaves to Darkness:
In the incalculably distant past, the World was visited by a starfaring race known as the Old Slann. Their degree of scientific advancement caused some of the species they met to worship them as gods, while others reviled them as demons. The Old Slann performed many scientific experiments on the World, and although the knowledge of their presence is lost in the present day, many of the races which inhabit the world found their origin in these experiments.
The Old Slann travelled by means of interdimensional gateways, spanning the distance between stars by traveling through ‘warpspace,’ a parallel dimension which connects all points in the material universe. One of the first tasks upon arriving on the world was to set up a pair of gateways, one at each pole of the planet, to allow them to come and go as they pleased.
Warpspace, however, was not an empty void, but was composed of a form of power wholly alien to the material universe. It was inhabited by entities who were equally alien. As their great ships travelled through warpspace the Old Slann protected themselves with powerful enchantments, but eventually something went dreadfully wrong.
Precisely what happened can only be guessed. Perhaps the protective enchantments broke down, or the beings that lived in warpspace found a way to overcome them. Or perhaps the gateways broke under the strain of the magical forces that cycled continually through them. Whatever the cause, the gateways collapsed. Both poles were destroyed and permanent dimensional tunnels were created between the World and the void of warpspace.
Among the matter sucked through into the World is warpstone, a substance formed of condensed and solidified essence of Chaos. Warpstone dust rained down upon the World at the time of the catastrophe, twisting many races into strange and horrible shapes. The fallout of warpstone dust was also the catalyst that stimulated the development of the Human race, giving it the versatility and lust for change that led to the present Human dominance over many parts of the world…
Many beings from the void chose to send a part of their substance to the World. Some were comparatively small and weak, while others were possessed of almost incalculable power. Thus were born the gods of law and chaos and their daemonic minions. The natural laws of the material universe affected their alien substance, giving them forms and powers which they had not possessed in their home dimension, and they found that they were able to hold sway over the inhabitants of the World, and to use for the furtherance of their own unfathomable ambitions. There was a constant struggle between the followers of Law and those of Chaos. The gods of Law were few and comparatively weak, but heir unity made up for their lack of numbers.
Seven thousand years later, the battle between Law and Chaos still rages across the face of the world, with most of its mortal inhabitants caught helplessly in the middle. The dimensional openings at the poles change constantly, shrinking with the ascension of Law, and growing as Chaos gains the upper hand. All the while, a constant stream of raw Chaos floods fro the void.
For millennia, the tide of chaos has ebbed and flowed across the face of the world. The worst incident in recent history was the great Incursion of Chaos 200 years ago, when Norsca was completely overrun and the Chaos hordes ravaged the northern parts of the Old World…
And in warpspace, the primal void of Chaos, the beings of Chaos still wait and spread their taint across the Imperium and the universe. Warp creatures hover around the vulnerable psykers of humanity, seeking a path into existence through an unprotected mind. Every psychic is a potential gateway from the void of the Warp, an unwitting agent of Chaos to be filled with terrible power.