The birth of the entities later known as the Star Gods occurred at the same time as the moment of Creation itself, as they formed from the vast, insensate energies first unleashed by that churning mass of cataclysmic force. In that anarchic interweaving of matter and energy, the sea of stars began to swirl into existence and for an eon the universe was nothing more than hot hydrogen gas and light elemental dust ruled over by the gravitic force of billions of newborn suns.
Long before the first planets had formed and cooled, the very first truly self-aware beings emerged into the universe, their thoughts encased within the lines of force produced by the plasma and electromagnetic flares of the stars themselves. In later times, these entities would become known as the C'tan, but early in their existence they were nothing like the malevolent beings they would eventually become. They were little more than monstrous energy parasites that suckled upon the solar energies of the stars that had brought them into existence, shortening the lives of otherwise main-sequence stars by millions of standard years.
In time, these star vampires learned to move along the lines of the universe's electromagnetic flux, leaving their birthplaces to drift through the cosmic ether to new stellar feeding grounds and begin their cycle of stellar destruction once more. Beings of pure energy, they paid no mind to the hunks of solid matter they passed in the vacuum of space, the blazing geothermal fires and weak geomagnetic fields of these nascent planets insufficient to be worth feeding even their ravenous hunger.
Just as the stars gave birth to their children so the planets of the newborn Milky Way Galaxy eventually gave birth to lifeforms composed of matter which began the long evolutionary climb to self-awareness. The first intelligent beings of the galaxy known to have developed a civilisation technologically advanced enough to cross the stars was a reptilian species of beings later called the "Old Ones" by the Aeldari, who knew them best of all their creations.
The Old Ones possessed a slow, cold-blooded, but still deep wisdom, having long studied the stars and raised astronomy and physics to such a level that their science and technology appear now to Humanity like an arcane art. Their understanding of the workings of the universe were such that they could manipulate alternate dimensions and undertake great works of psychic engineering.
The Old Ones were potent psykers who routinely used the powers of the Warp for a wide variety of technological applications. This included the construction of a system of instantaneous faster-than-light portals through Warpspace. These portals connected all of the Old Ones' colony worlds, allowing them to cross the vast gulfs of space with only a single step through the myriad Warp Gates they built to connect the worlds of the galaxy. This was a vast interdimensional network much like the Webway of today which is its direct descendant, though on a much larger and more advanced scale.
Through this network the Old Ones spread their spawn to many places in the galaxy, but they also knew that all life was precious. Where they passed, they seeded new intelligent species and reshaped thousands of worlds to make them their own according to their predetermined environmental and geographic criteria. It is believed by some in the Adeptus Mechanicus that even Terra felt the Old Ones' touch long before Humanity's rise to self-awareness, though this notion is considered heretical at best by the Ecclesiarchy, as the Imperial Creed teaches that Mankind was made in the image of the God-Emperor before His spirit was incarnated in physical flesh millennia ago.
The Old Ones' civilisation reached its height in excess of 60 million standard years ago. The Old Ones were responsible for the creation or genetic advancement of most of the currently active intelligent species of the galaxy, including the Aeldari, the Krork (the Orks' precursors), the Slann and the Jokaero, though it is unknown if they played any direct role in the evolution of Humanity.
The humanoid species that would become the Necrons began their existence under a fearsome, scourging star in the far reaches of the galaxy known as the Halo Stars region, billions of standard years before Humanity evolved on Terra. Assailed at every moment by solar winds and intense radiation storms, the flesh and blood species known as the Necrontyr became a morbid people whose precarious life spans were riven by constant loss. What little information the Imperium of Man has recovered on the Necrontyr tells that their lives were short and uncertain, their bodies blighted and consumed at an early age by the terrible cancers and other illnesses linked to the high levels of ionising radiation given off by their sun.
Necrontyr cities were built in anticipation of their inhabitants' early demise, as the living were only brief residents scratching their daily livelihoods in the shadow of the vast sepulchres and tombs of their ancestors. Likewise, their ruling dynasties were founded on the anticipation of an early death, and the living were thought of as no more than temporary residents hurrying through the more permanent and lasting structures raised to honour the dead. On the Necrontyr homeworld, the greatest monuments were always built for the dead, never the living.
Driven by necessity, the Necrontyr escaped their crucible-prison and struck out for the stars, hopeful of carving out an empire in which they could realise their species' freedom from the lethal energies of their birth star. Using stasis crypts and slow-moving, antimatter-powered torch-ships that were clad in the living metal known as necrodermis to resist the millennia-long journeys through the void, the Necrontyr began to colonise distant worlds.
Little by little, the Necrontyr dynasties spread ever further, until much of the ancient galaxy answered to their rule. From the earliest days, the rulers of individual Necrontyr dynasties were themselves governed by the Triarch, a council composed of three Necron dynasty monarchs known as phaerons. The head of the Triarch was known as the Silent King, for he addressed his subjects only through the other two phaerons who ruled alongside him. Nominally a hereditary position, the uncertain life spans of the Necrontyr ensured that the title of Silent King nonetheless passed from one royal dynasty to another many times. The final days of the Necrontyr Empire occurred during the reigns of the Szarekhan Dynasty, the last of the Silent Kings.
Sometime during their slow interstellar expansion, the Necrontyr encountered an ancient species far older than any other in existence in the known galaxy. These beings were the Old Ones, and they were absolute masters of forms of energy the Necrontyr could not even conceive of, yet alone wield. The Old Ones had long ago conquered the secrets of immortality, yet they refused to share the gift of eternal life with the Necrontyr, who yet bore the genetic curse of the bitter star they had been born under.
The colonisation of much of the galaxy by the reptilian mystics had been immeasurably swifter and more expansive than that of the Necrontyr because of their Warp Gates and mastery of the Immaterium. That, and the Old Ones' incredibly long, if not downright immortal lifespans, kindled a burning, jealous rage in the Necrontyr, which ate at their culture spiritually as much as physical cancers consumed their bodies. The Necrontyr were astonished to learn that another intelligent species enjoyed such long lives while their own were cut so brutally short.
But as time wore on, further strife came to the Necrontyr. Each dynasty sought to claim its own destiny and soon the great houses were engaged in all-out conflicts known as the "Wars of Secession." Had circumstances remained as they were for but a generation more, it is possible that the Necrontyr would have wiped themselves out, as so many species had before them and shall do in the future. As their territory grew ever wider and more diverse, the unity that had made them strong was eroded, and bitter wars were waged as entire realms fought to win independence.
Ultimately, the Triach realised that the only hope of unity for their people lay in conflict with an external enemy, but there were few who could prove a credible threat to the technologically advanced Necrontyr. Only the Old Ones were a prospective foe powerful enough to bind the feuding Necrontyr dynasties to a common cause. Such a war was simplicity itself to justify, for the Necrontyr had ever rankled at the Old Ones' refusal to share the secrets of eternal life.
So it was that the Silent Kings of the Szarekhan Dynasty, in a typically bitter act of jealousy and resentment for the Necrontyr species, declared war on the Old Ones. At the same time, he offered amnesty to any secessionist Necrontyr dynasties who willingly returned to the fold of the Triarch. Thus lured by the spoils of victory and the promise of immortality, the separatist Necrontyr realms abandoned their Wars of Secession and what was later known as the "War in Heaven" began. A conflict unlike any fought before erupted across the stars, yet while the Silent King and the Triarch had succeeded in uniting their hateful people, it was a war the Necrontyr could not win. Not on their own.