The Void

In the beginning, the void was grey, perfectly spherical and at the same time practically infinite. 
However, on closer examination, it was not actually grey, but a mixture of minuscule particles that were different and could perhaps be considered as being black and white. The particles were totally fluid yet so numerous that they were close to solid. The particles collided, ricocheting outwards in near-random directions, black and white particles swirling continuously, perfect chaos. 
However, each movement and ricochet was governed by rules, and the swirling dance of particles never changed, never altered, in a way perfectly static and ordered.
The void was sentient and self-aware. It was an incredibly large single consciousness. Its thoughts taking millennia to form and stabilise. For untold aeons it considered its nature and its reason for existing. Nothing changed and it was content.
Then something changed. A small bubble formed on its extremity.  A tunnel emerged through its midst, a tunnel which did not form part of itself and did not belong. The tunnel grew and extended into the void and upon reaching another of its extremities the tunnel was complete. 
The voids thoughts were disturbed. Where before perfect unchanging chaos had existed, now ripples and eddies slowly swirled throughout the void. The ripples and eddies imposed a new type of order on the chaos and at the same time caused a change, interrupting the perfect order that had existed.
Something about the interaction between the tunnel and void created friction introducing a slight charge along their connecting surfaces. Ever so slowly white particles began drifting toward one entrance of the tunnel and began to rotate clockwise. The black particles drifted to the other and began to rotate anti-clockwise. The void was so large that it's middle remained as it had been, a fusion of black and white which did not revolve.
The grey in the middle of the void was initially unconcerned about the intrusion. It continued contemplating as it always had. 
Then suddenly near one end of the tunnel, the white particles began an independent thought. They wanted to return everything back as it had been.  They knew that the tunnel had changed their nature. They wanted the tunnel to withdraw, so that things could return to how it always had been. 
In the same time, another thought spread through the black particles. The tunnel was interesting, something new to contemplate after aeons of sameness. Where a single thought had been, in an instant there were now three.
Another bubble appeared, another tunnel extended across the void and terminating at a new point. A new charge introduced to the void. The swirling intensified, became faster, thoughts that had taken aeons now only took centuries. The swirls and eddies caused by the new tunnel slowly stabilised into their own rotating collections of particles like galaxies in the physical universe.
Each swirl of particles consisted of differing numbers of black and white particles. Where three thoughts had been, now there were numerous thoughts all different all conflicting.
Another tunnel appeared, and another, followed by yet more. The swirling thoughts of the void multiplied exponentially, intensified, became more focused. Many contained smaller similar swirls within and they becoming more than just a single thought, they themselves became conscious.
The new entities examined their surroundings and the tunnels that traversed through their void. They became aware of the small objects passing through these tunnels, and they sensed many small consciousnesses within these objects. They studied and communicated with these consciousness's, who revealed themselves as either The Slann, The Old Ones or sometimes even as The Star Gods.
Many of the entities who were mainly white still wanted a return to how things had been in the beginning and now actively plotted how to destroy the tunnels, in the hope that they could restore the old order. The other white entities had accepted the current conditions and wanted to ensure that the current order was maintained. Each entity had different ideas about how to achieve their goals.
However, the entities composed of mostly black particles had savoured the upheaval the tunnels had created, they enjoyed studying the new developments and thinking new thoughts. As they reached conclusions on the new nature of things they began to crave further change to spur further new thoughts. Some thought to create change by creating new tunnels or altering the original independent tunnels to intercept. Others thought that the destruction of the tunnels would propagate further new developments.
The entities that consisted of similar numbers of black and white particles continued to contemplate the nature of the order that now existed. Some of these entities even discovered how to seep into the tunnels without being destroyed and at least for brief time to emerge into the physical universe outside the void and to communicate directly with the travellers from the tunnels in their own environment. These entities became known to the mortals as the old gods. They examined their new surrounds and took on whatever aspect seemed appropriate from their new surroundings stimulated and guided by the needs of the mortals they were visiting.
For a while, everything within the void remained the same. The void was in chaos as the many entities considered their many thoughts and carried out their plans, yet order still ruled as the plans and actions of each entity changed little and a natural harmony amongst the whole developed.
Then something changed again. Was it due to the actions of a black entity or of a white entity, or even something from outside the void from where the tunnels had been created? 
There were new travellers entering the tunnels now who no longer called themselves Star Gods, or Slann or The Old Ones and their thoughts were unguarded and powerful.  As these new travellers moved through the void they attracted an ever-growing following.  The entities in the warp pursued these travellers competing for their interest and attention and growing ever more powerful themselves as they coalesced around these new exciting stimuli.
Competition and conflicts began to erupt between the entities in the warp as some entities sought to dominate access to the travellers and others tried to deny them access. Power and growth became dependant upon access and availability to the most powerful thoughts and the strongest emotions and the warp entities began to sift through the wide variety on offer and to focus on those they found most appealing to them and their collective. 
As competition grew the warp entities became more adept at stimulating the thoughts and emotions they had chosen to support and began not just to listen and communicate with the travellers but to try to interact with them and stimulate an increase in the emotional response they got in return.  Now travelling in the void became much more hazardous as warp entities hunted the travellers along the tunnels desperately trying to maximise the output from each encounter in order to grow stronger and fight off the competition for future access.
The void was no longer calm and orderly but was now a bubbling cauldron of storms and conflicts, confusion and chaos.
Then there came a final change when one group of the travellers calling themselves The Elder stopped travelling through the tunnels and instead stopped and opened their minds to unlimited access to their chosen Warp Entity.  It's name was Slaanesh and it had chosen wisely to feast on thoughts of self-indulgence, pleasure and personal gratification and The Eldar had formed cults specifically to worship and pursue those very things.
The pleasure cults had also begun to compete with each other for access to the wonders that only warp energy could provide and their own hunger for pleasure mirrored the desire of Slaanesh to feed on the power their desires provided it.  It rapidly grew more powerful forcing its small rival entities to one side and filling the void as more and more particles were subsumed into it's being.  It's hunger for power subsumed other lesser emotions such as love, lust, passion, greed, even minor emotions were sucked into the whole like pride, avarice, vanity and happiness.
Slaanesh grew rapidly consuming everything it could tolerate and grew so large and powerful that it filled the void and began to put pressure on the very boundaries of the immaterium.  But still, the travellers came seeking the ultimate pleasure experience, still Slaanesh provided it and still it was not enough.         The final act came when engorged beyond the ability of the void to contain it Slaanesh finally burst out of the Void into material space rupturing the gates at the end of several tunnels and burst into the real world. In a desperate search for more power, Slaanesh erupted into the real world and consumed everything it could find that appealed to its taste.  Eldar Craftworlds which were situated close to some gates were consumed in an instant sucked dry and swallowed whole.  Nothing in the void at the time survived and nor did anything beyond if Slaanesh found it appealing.
As each gate collapsed the particles closest to it were expelled into the material universe beyond and as the void material became subject to the laws of reality most of it was transformed into some sort of solid object drawing on whatever stimulus was available to inspire its form and where none was available simply coalescing into rock, dust or crystals.  
One set of Gates terminated at a planet named 'Warhammer'. The tunnels here had terminated above the planet's poles and the warp energy that erupted from one of these was vomited out into material space where unable to find any stimulus it eventually formed an orbiting sphere and later became known as the chaos moon Morrslieb.
While the void energy from the planets other gate was vomited towards the planet raining down on the surface as meteors, rocks and dust or being blown across the planet in the wind where it took whatever bizarre material form of which it became aware.
This material became known as warpstone and reacted badly with the natural material of this universe.
The planet was luckier than most worse gates had collapsed. There were no craft worlds on the planet and so Slaanesh did not pursue it's hunger through the rupture gates and scour the surface clean of mortal life.  Nevertheless, huge quantities of warp energy did erupt through its gates contaminating the planets eco-system and reacting violently with any emotions it found most appealing.
Fortunately, most of this material was rendered inert, if not exactly destroyed, as it was forced to take physical form before it could find a suitable stimulus to maintain itself. But the gates continue to seep warp energy creating what academic's refer to as the winds of chaos.  However, it is only partially destroyed and can still be stimulated back into life by exposure to the right emotional signals.  
The moon Morrslieb is a large piece of warpstone, special in both its mass and density. Morrslieb's physical composition acts as a medium to conduct the remaining thoughts of the void entities who's substance created it. Within the physical material of the moon thoughts sought like thoughts and combined into several large and many smaller intelligences. They reached out to the consciousness's they felt on the planet below and moulded themselves on the primal fears, hopes and emotions they discovered there, and eventually became known as gods.
Unfortunately these intelligences were tainted by what they had been. Many sought to impose their form of order or stasis on their new universe, as they had in their old. These intelligences became known as the gods of law. Others still sought change and strove to create it in one form or another. These intelligences became known as the gods of chaos.
The remaining intelligences where content contemplating their new existence and ensuring that neither group of their kin could gain dominance. These became known as the neutral gods, or the gods of balance. As things had been in the void, they now become within Morrslieb.


>From the forbidden book of what was and is One of the great secrets revealed by our lord Tzeentch As recorded by Aaron Pile