Driven to perfect their science of proto-genetic evolution, the ancient, enigmatic race known as the Xel'Naga traveled to the distant fringe-world of Aiur. The vast jungles of Aiur had produced the most advanced race that the Xel'Naga had ever seen. Believing that they could steer the race's evolution to the pinnacle of physical perfection, the Xel'Naga began to conduct their proto-genetic experiments. The race that the Xel'Naga would eventually name "Protoss" advanced rapidly and gained what their creators termed "a distinct purity of form".

Unfortunately, the Xel'Naga pushed their experiment too far. The inherent essence and sentience of the Protoss developed far too rapidly, leading to bitter strife and division between them and their creators. The Xel'Naga deemed that the purity of form they sought to create had been sullied by a conflict of essence and thus decreed that the Protoss were, in fact, a failed creation. The Xel'Naga abandoned their children and launched themselves into the void.

Traveling thousands of light years into the burning core of the galaxy, the Xel'Naga eventually settled upon the volatile ash-world of Zerus. The Xel'Naga planned to continue their Grand Experiment of evolution, only this time they dismissed their tenets of physical and focused chiefly on the pursuit of a distinct purity of essence. Residing in their massive ships high above the fires of Zerus, the Xel'Naga began once again to challenge the wiles of fate.

The Xel'Naga were more successful with their second venture than they could have imagined. They laboured to advance the evolution of the most insignificant life form on Zerus, a race of miniature insectoids known as the Zerg. Through Xel'Naga proto-genetic manipulations, the Zerg survived the torrential firestorms of their world and thrived. Although extremely small, worm-like, and possessing no ability to manipulate their physical surroundings, the Zerg adapted to survive. They developed the ability to burrow into the flesh of the less vulnerable species indigenous to Zerus. Feeding off the nutrients contained within the spinal fluids of their hosts, the Zerg learned to parasitically merge with their host creatures. Once they became capable of controlling the metabolic and anatomical processes of their hosts, the Zerg used their new bodies to manipulate their surroundings.

As the Zerg incorporated more and more host creatures into their fold, they began to assimilate their various genetic strains and processes. Zerg chemistry began to mutate and adapt according to the volume of new genetic material being processed. However, as diverse as the range of host creatures became, there was always the undeviating drive to consume only the most evolutionarily advanced species encountered. The Zerg were innately selective as to which species they consumed, ensuring that at every stage of their development they were at the top of the proverbial food chain. Any race that the Zerg came across that was deemed unworthy of assimilation was eradicated to further purify the strains.

The Xel'Naga soon made an alarming discovery. The original races assimilated by the Zerg were hardly recognizable after only a few generations of their inception. Somehow the Zerg had developed the ability to supercharge and steer the latent evolutionary processes within their host creatures. The host creatures fell prey to the effects of gradual physical mutations that caused all of the various strains to grow armor piercing spines, razor-sharp limbs, and ultra dense carapaces. Over a surprisingly short amount of time, the strains grew to resemble a terrifyingly ravenous and unified race.

The Xel'Naga, remembering all too well that their failure with the Protoss was a result of pushing the sentience of the fledgling species too quickly, decided to follow a different path with the burgeoning Zerg. Attempting to waylay the potential hazards of differing egos, the Xel'Naga structured the collective sentience of the Zerg into a unified, amalgamated "Overmind". The Overmind coalesced into a semi-sentient being that represented the primary drives and instincts of all of the Zerg strains. As time passed, the Overmind developed the rudiments of personality and advanced intellect.

Although the Overmind directed the actions of every creature within the swarm, it did so through the use of secondary agents. The Overmind created a new strain of Zerg that could facilitate the communication of its whims. These Cerebrates were massive versions of the original Zerg insectoids and were engineered to process and carry out the various objectives of the Overmind. Each Cerebrate had a unique objective, such as "Defend The Hive", "Scout For Potential Strains", "Produce New Warriors", or "Eradicate All Life Forms". The Cerebrates were then empowered to build their own Hive clusters and carry out their intended agendas. Over time they developed personalities that corresponded to their primary drives. However, the Cerebrates were still genetically incapable of ignoring or overriding the commands of the Overmind.

The Zerg chain of command lengthened as new races were incorporated into the Swarm. Cerebrates began to use secondary agents to communicate their orders as well. For the defence and productivity of the immediate Hive, the Cerebrates created Queens to further extend their control over the swarms. The Queens oversaw Drone activity throughout the resource lines of the Hives and kept a vigilant watch over the budding spore colonies of the collective. In times of battle, Cerebrates called upon the Overlords to directly relay their commands to the myriad of warriors of the Swarm. Overlords not only transported Zerg warriors into the fray, but also directed their attacks against the enemies. Just as the Cerebrates followed the Overmind, the Queens and Overlords were incapable of disobeying the commands from their Cerebrates. This rigid system of command kept the swarms at peak, savage efficiency.

As the swarms continued to grow and strengthen, the Overmind turned its thoughts towards its own future. It realized that within a few short centuries its race had assimilated all of the indigenous life upon Zerus. It knew that to further evolve the swarm, the Zerg would need to leave Zerus. The Overmind began to reach out with its senses, looking for something - anything - which would provide them with transport from this world. That opportunity soon arrived. A race of gargantuan, space-faring life forms passed through the Zerus system, and the Overmind called to them. Drawn by this beacon, they were quickly assimilated by the swarm. The inclusion of super-dense hides and the ability to exist in a vacuum bolstered the genetic pool of the swarm. Soon the Zerg warriors were conditioned to survive the harshness of space.

This pivotal moment in the development of the Zerg was not lost to the Xel'Naga. The Zerg, despite having an extreme physical handicap, had succeeded in not only surviving, but in retaining the purity of their terrible overriding essence. The Xel'Naga knew that they had achieved their goal.

The pride in their achievements proved to be the fatal downfall of the Xel'Naga. The Overmind, while slowly expanding itself into the void of space, became aware of the mighty Xel'Naga world-ships hovering ominously above the skies of Zerus. The Xel'Naga, having kept a constant watch on the Overmind, were horrified to find that it had actually severed their psychic link, effectively hiding itself from their view. With its need to consume driving its minions into a lustful frenzy, the Overmind launched the now space-faring Zerg swarms at the unsuspecting Xel'Naga. The ancient race did what they could to stem the tide of the ever-advancing Zerg onslaught, but in the end their efforts were in vain. Wave after wave of Zerg swarms hammered the reinforced hulls of the Xel'Naga's ships with no signs of abatement. Within only a few hours the Zerg overran the defenses of their creators and laid waste to the Xel'Naga fleet.

As the greater whole of the Xel'Naga race was consumed by the raging, genetic whirlwind of the Zerg, the Overmind gained the knowledge and insights of its masters. The Overmind processed thousands of sentient beings into itself, causing it to grow far more powerful than it had ever imagined. It learned the secrets of the sacred Khaydarin Crystals, and began to incorporate the energies of these Crystals into its own. Through the intimate knowledge of evolution and proto-genetic physiology gained from the Xel'Naga, the Overmind was able to increase the level of sentience in many of the higher Zerg strains, while still keeping them fully under its control.

Through dissecting the memories of the Xel'Naga, the Overmind was made aware of the myriad races that had at one time or another been influenced by the ancient race. The Xel'Naga had kept a detailed genetic history of each race, giving the Overmind a clear understanding of their respective strengths and weaknesses. Most importantly, the Overmind learned of an exceedingly powerful race that lived near the galaxy's fringe known only as the Protoss. The Overmind knew then that the Protoss and the Zerg would be caught in an inevitable, apocalyptic conflict.

The Zerg left the lifeless, burning world of Zerus and laid waste to every planet they found along their path towards the Protoss Homeworld. As they progressed slowly through the trackless dark between the stars, the Zerg assimilated only the strongest of the races that they came across. The swarm continued to build steadily, ever-increasing in size and power. As they progressed, the Overmind sent out numerous deep-space probes that scouted ahead of the swarm, searching for new worlds to plunder.

Despite innumerable victories, the Overmind was greatly disturbed. The Overmind was aware that the Protoss had become a highly psionic race, able to bend and warp the very fabric of reality to their whims. It sought a way to counter the awesome might of the Protoss, but found no answers among the genetic strains it devoured.

On the verge of despair, the Overmind made an amazing discovery. One of its deep-space probes had relayed the location and vital statistics of a race that occupied a series of nondescript worlds right under the shadow of the Protoss.

The new race, called Humanity, was mere generations away from developing into a formidable psionic power. But the Overmind also knew that Humanity was still in its infant stages, hardly capable of defending itself against the ravenous Zerg. Although a short-lived and seemingly frail species, the Overmind knew that Humanity would be the final determinant in its victory over the Protoss. If it could assimilate the psionic potential of Humanity, the Overmind would have the ability to combat the Protoss on its own terms.

Thus, the Zerg swarms slowly made their way towards the burgeoning worlds of Humanity. The journey lasted for sixty years, but eventually the massive, extended Zerg Swarm reached the outskirts of the Terran sector of Koprulu. Sending in a scouting Brood, the Overmind soon discovered that the Terrans dwelt on over a dozen different worlds within the Sector. Seeding the atmosphere of the planet called Chau Sara with rudimentary Hive-spores, the Overmind began to unfold its master plan to enslave Humanity. The Hive-spores gradually drifted down to the surface of Chau Sara and saturated the ground with their denuding, alien toxins. Although the Human colonists had no idea that the Zerg had subtly infested the topsoil of their world, Zerg minions began to descend to the planets surface and construct their bizarre structures and Hive clusters. Once the infestation of the colony was well underway, the Overmind sent its voracious children to the other nearby worlds. Insidious and elusive in their tasks, the agents of the Overmind soon infested the colonial worlds of Chau Sara, Mar Sara, Brontes and Dylar IV, unnoticed by the denizens of those colonies.

Yet, from out of the cold void of space, a mighty fleet of Protoss warships emerged to combat the Zerg invasionary forces. The Overmind, anxious to learn what it could about the enigmatic Protoss, decided to let them hamper the initial infestation process. Holding its ravenous warriors at bay, the Overmind watched as the Protoss razed the colony of Chau Sara. Apparently, the Protoss were aware that the Hive Spores had already despoiled the planet and seeking to prevent further infestation, incinerated the planet.

Such decisive action pleased the Overmind, who could only marvel at how the Protoss wrought absolute devastation with such grace and power. Knowing that the coming conflict would be the greatest challenge of its existence, the Overmind pulled its forces back to observe how the Protoss and Humanity, would react next.

After the first world the Zerg took from the Terrans was cleansed by the Protoss, the Overmind realized that the target of its search was near and it became determined to locate the Protoss home world as quickly as possible. To this end, the Overmind sent most of its minions away from Char to scout the area. The Overmind also began to experiment with a very powerful Human psychic named Sarah Kerrigan. Kerrigan was subjected to a very special and extensive infestation and as she developed, she was brought to Char for closer observation. It was during this time that Tassadar arrived on Char with his forces, so that he could learn more about the Zerg and the Overmind.

While on Char, Tassadar located a member of a rouge Protoss group called the Dark Templar, and made contact with them. Together Tassadar and the Dark Templar leader Zeratul determined that in order to permanently kill a Cerebrate, and eventually the Overmind, the magical energies that the Dark Templar studied would be needed. Together, the two launched an attack on one of more important Cerebrates in the Zerg Swarm and permanently killed it. The other Cerebrates were terrified, as they normally do not fear death, but the Overmind was happy beyond measure, for in the moments before the Cerebrate was killed, Zeratul was forced to make mental contact with the Cerebrate, and during that contact, the Overmind was able to discern the location of Aiur, the home world of the Protoss.

With the goal in reach, the Overmind abandoned the Terran Confederacy and nearly the entire Zerg Swarm was brought to Aiur. To this end, most Zerg located on planets throughout the sector were collected and sent to either Aiur or to Char, where the Overmind wanted to keep his forces not directly involved in the invasion. All of its most important Cerebrates were brought to Aiur to coordinate the invasion, but Kerrigan was left on Char to look after the remaining Cerebrates and Zerg creatures located there.

The first step in the invasion was to bring the Zerg Swarm to Aiur and to eradicate the single greatest threat to the invasion, the Protoss Grand Fleet. Using its immense magical abilities, the Overmind created a massive dimensional portal that engulfed the Grand Fleet and sent it to another dimension. Knowing very little about dimensional travel, the Overmind did not care where the fleet went, only that it was no longer able to protect Aiur. The Overmind then sent several large armies to Aiur to secure an area large enough, and with a large enough supply of Khaydarin Crystals, to physically manifest itself on Aiur, as eliminating the Grand Fleet drained most of the Overmind's magical reserves. Within the first few days of the invasion, the Zerg were able to overwhelm the Protoss defenders in several key areas, which enabled the Overmind to physically travel to Aiur to personally oversee the invasion.

The Protoss fought valiantly against the overwhelming odds and the Conclave even believed it was winning the war on several occasions. The Zerg Swarm spread across the planet, decimating city after city and defeating army after army. As the remaining Protoss forces tried to regroup for a counter offensive, the Protoss Executor Tassadar returned to Aiur with a small force of Dark Templar, Terrans from the Terran Confederacy, and his army of loyal Protoss warriors. Together with a large force of Protoss warriors under the control of local commanders, Tassadar mounted a daring assault on the Overmind directly. At the height of the battle, Tassadar, having closely studied magic with the Dark Templar, merged magical energies with psychic energies in the core of his flagship and rammed the Overmind's body. The resulting explosion not only killed Tassadar and the Overmind, but also a large section of the Zerg Swarm immediately surrounding the Overmind.

With the Overmind dead, the Protoss hoped that they would be able to easily win the battle and reclaim their home world, but the remaining Zerg Swarm rampaged uncontrollably over Aiur, killing anything in their path. Most of the surviving Protoss were forced to flee from their home world to avoid the swarming Zerg.

The surviving Cerebrates on Aiur became independent and began to fight amongst each other for dominance of the race. The Zerg in other parts of the galaxy became independent and were completely cut off from the rest of the race; only the Overmind knew how many Zerg were not on Aiur or Char. Several Cerebrates on Aiur followed the escaping Protoss to complete the last mission that the Overmind gave them before its death, but a few Cerebrates, both on Aiur and on Char, began to develop new strategies to deal with the situation. On Char, several Cerebrates devised a magical ritual that would allow them to combine their powers and create a new Overmind, and they would have been successful, if it hadn't been for the involvement of the Terrans from Earth and Sarah Kerrigan, who had her own plans for the Zerg Swarm.

Sarah Kerrigan, now calling herself the Queen of Blades, manipulated the Terran Dominion, the successors of the collapsed Terran Confederacy, the remaining Protoss forces, the Dark Templar, and the United Earth Directorate, a powerful force of Terrans from Earth, into serving her. Within a relatively short amount of time, Kerrigan was able to gain control of a huge portion of the remaining Zerg Swarm and dispatched the rival Cerebrates that wanted to take control of the Swarm. From her headquarters on Char, the Queen of Blades had a powerful army and little in the way of threats; her dreams of conquest were only just beginning to take form.