An attempt to reconcile the ways of the old with the passions of the new:
As humanity first began its journey down the path of sentience, the species' reflection in the empyrean began to grow. Souls ignited with passions and trains of thought never before seen around Earth. Soon, individuals with spirits of impressive potency appeared, such that they could see, feel, and influence the energy of the aether on rudimentary levels. These men and women would come to be referred to by many titles: witches, warlocks, shamans, etc.
These shamans understood the true nature and horror of the warp. Almost without exception, the shamans guided their peoples and protected them from the predations of the occasional warp-hunter that sought to feed on them.
Though the shamans possessed powers beyond the ken of normal men and women, they were still first and foremost human. Their lifespans could not be extended for any great measure, and they died like those around them. Their souls entered the warp and began to coalesce, drawn together by their mutual need for protection and their desire to protect human kind. Over the span of thousands of years, shamans from all over the world died and found themselves drawn to the collective. As one, they fought warp-spawned beasts and shielded the species from the attention of the Dark Gods. With such single-minded purpose and purity of focus, it did not take long (by measure of the timeless warp, of course) for a singular consciousness to form. Thus the Starchild was born.
The Starchild's powers were incredible. The entity's nature balanced and quenched the chaotic fury of the warp's energy in its vicinity. It's touch could cleanse mutation and cure madness. It spoke to newer generations of shamans, giving them healing powers and prescient wisdom, and guiding them to its side when they left the mortal world. Perhaps its strangest and most magnificent power was to keep itself invisible- hidden from the gaze of Chaos, the only power in the galaxy strong enough to tear it asunder.
As humanity began creating civilizations and abandoning shamans and the need for them, it also began exploring its own psyche more. While this brought the species to greater philosophical and scientific heights, it freed the darker side of the mind to flirt with dangers and uncontrolled passions. With fewer shamans passing on their lineage and souls becoming more difficult to manage, the Starchild found its power plateauing, its chosen task increasingly daunting, and itself running out of options. Mankind needed to be protected from itself.
To this end, the Starchild decided to begin influencing human affairs directly. It came to all remaining shamans and called them together. It asked them to join it, for the sake of humanity. The shamans engaged in a mass suicide, and fed the Starchild with an influx of new power. As part of the plan, one shaman from the region known as Anatolia left his wife pregnant with a son. This boy would be a vessel for the Starchild. He would have his own consciousness and identity, but he would also have a special connection to the Starchild, so that he might utilize its wisdom and carry out the entity's perrogative.
This boy would grow to become a man, and he wielded tremendous power. He was functionally ageless and possessed the ability to remake reality as he saw fit. For thousands more years he watched his species grow and experiment. He spent ages learning philosophies and sciences, and shared pieces of his considerable intellect to better the people. At times he took control of a sect of mankind directly, guiding it along paths to set examples that would inspire. On other occasions, he worked behind the scenes, guiding world leaders and trying to influence humanity to work to better itself. But the passions of the human soul proved too restless to be manipulated so discretely.
Humanity spread throughout the stars on its own, taking much of its own beyond the grasp of the Starchild. The Dark Gods, as well as many alien races, began taking interest in the potential of humans, for balance or for Chaos. The Age of Strife took hold, as warp storms cut off huge populations from contact with Earth, and a great many people were slaughtered. Through its relentless pursuit of economic and scientific opportunity, mankind brought itself into its darkest days.
The "New" Man, and indeed the Starchild itself, eventually realized that more direct action was needed. Using unpracticed sciences and the reailty-warping power of the aether, the New Man created himself a more formidable and potent body. He recruited, taught, and learned from the greatest genetecists, arms manufacturers, and logisticians of the day and began a conquest of the globe. With genetically engineered super-soldiers armed with the finest weapons and armor known to man at his side, he marched forth and took control of Earth. The New Man, having tasted the limits to his power and the scope of what he sought to accomplish, turned his steely gaze to the stars. And thus, the Great Crusade began.