The Zerg are no longer a single species, but many different species under the single control of the Overmind, a highly advanced creature created thousands of years ago on a scorched planet. The Xel’Naga created the Overmind and the original Zerg creature after they created and abandoned the Protoss. The Xel’Naga viewed the Protoss as a failure, as they were not perfect in every way their creators wanted. While the Protoss embodied perfection of body, the Xel’Naga wanted more from their creations, so they deserted the Protoss and traveled to desolated planet where they discovered a simple life form that had survived the planet-wide ecological disaster that had killed nearly all other life. Seeing this life form as already very adaptable, the Xel’Naga believed they could focus all of their energies into creating a species that embodied a “perfection of spirit”.
To control their new race, the Xel’Naga created the Overmind, a large creature that, while it had no outward defenses, could completely control the new Zerg race. The Overmind began to learn at an amazing pace, and quickly began to adapt the Zerg race to better survive in its environment. Unfortunately, the more the Overmind learned, the more it came be see the Xel’Naga as rivals. One day, the Overmind directed its race to attack the Xel’Naga in an attempt to kill them. The Xel’Naga were forced to flee from the planet, but not before they lost many of their numbers to the primitive Zerg race.
The story of the Zerg might have ended with the departure of the Xel’Naga. The Zerg possessed no technology, they could not leave their home world, and they could not chase down the Xel’Naga, which had retreated into space. But the Overmind would not be defeated; it had learned from the Xel’Naga that there was another creation out there, one that embodied the perfection of body. The Overmind wanted to find this race and assimilate its genetic code into the Zerg, creating the ultimate species that the Xel’Naga had envisioned.
Again, the story of the Zerg could have ended with the Overmind trapped on planet of its creation, with millions of loyal creatures under its control, but that would not come to pass. A random event changed the lives of the Zerg and the galaxy forever. A group of migrating space born life forms passed through the star system where the Zerg lived and the Overmind detected their presence. Because the creatures were unintelligent, the Overmind quickly dominated their minds and forced them to approach its planet. The creatures had the ability to enter and leave an atmosphere easily and survive in space and the Overmind wanted to possess those abilities. The entire group of space creatures was killed and its genetic code assimilated by the Overmind. Within a short span of time, the Overmind created the original Overlord creatures that would allow the Zerg to escape their home planet.
The Zerg Swarm left their home world and traveled through several nearby star systems at speeds slower than light, but the Overmind was patient and he knew that eventually his time would come. In a nearby star system, the Zerg discovered a planet rich in magical energies and ley lines and inhabited by intelligent creatures that understood the nature of magic. The Overmind had never seen such strange energies and over the course of several centuries, it assimilated the native population and learned to master magic. With its new magical powers, the Overmind was able to travel faster and farther then it had ever dreamed was possible.
With its new ability to travel through space, the Overmind and the Zerg quickly explored the surrounding space. Creatures they found were examined for potential use in the growing swarm, those found worthy were conquered, killed, and assimilated, while those found unworthy were ignored or exterminated if they were perceived as a threat to the Swarm. New species were added to Zerg Swarm and new subspecies were created and specialized by the Overmind, while old species and strains were abandoned. The Overlords were drastically improved over the originals; they were given limited intelligence, advanced awareness, and several other abilities so that they could act as mobile commanders in the Zerg Swarm.
Needing something more than commanders, the Overmind devised a means of creating a highly intelligent creature based on a combination of the Overmind’s own structure and on several recently assimilated species. The result were the Cerebrates, each of which would lead a portion of the Swarm, freeing the Overmind from such menial tasks and allowing it to spend its time reaching out into the universe to locate the Protoss and analyzing the assimilated DNA. The Cerebrates had their own personalities and sometimes they would even fight amongst themselves, but they never failed the Overmind, nor did they ever betray the Overmind, its control over them was as complete as its control was over the simplest Zerg.
Under the control of the Cerebrates, with the direction of the Overmind, the Zerg became a terrifying force and cut a path from one side of the galaxy to the other in their effort to locate the Protoss. In their travels, the Zerg became more and more efficient, as new species were assimilated and useful genetic material was integrated into their genetic codes by the Overmind.
One day, a Zerg deep space probe felt a presence reaching out to it with even more power and influence than the Overmind. The probe changed its direction to follow the call of the presence and inadvertently entered the Terran Confederacy, a small cluster of Human colonies and planets. An experimental device called the Psi-Emmiter, designed by the Terran Confederacy for completely different purposes, called out to the probe. When the Overmind realized that its control over one of its minions was being overwhelmed, it decided to investigate further. What it discovered was a powerful device that emitted energies both similar to its magical energies and yet completely different. The Overmind decided it would assimilate the race that had devised such a strange device and it prepared for an invasion of the Terran Confederacy. It began the invasion by infesting several outlying colonies with Creep Spores and established its main base of operations on the magical rich planet of Char, located near the Terran Confederacy.
The Overmind’s arrival near the Terran Confederacy was not unnoticed: a probe from the Protoss was also nearby; its mission to keep an eye on the Terran Confederacy, and it detected the sudden appearance of the Creep Spores. After careful analysis of the probe’s readings, the Protoss determined that the Creep Spores contained elements that could only have originated with the Xel’Naga, namely the Khaydarin Crystals. The Protoss then captured one of the Zerg deep space probes and using their psionic powers, they learned the history of the Zerg and their desire to assimilate the Protoss. Seeing the danger represented by the Zerg, the Protoss Conclave, the ruling body of the Protoss race, sent a fleet of warships to eradicate them, even if it meant killing the Humans do to it. The leader of the fleet, the Executor Tassadar, carried out his mission as swiftly as possible, but as more and more inhabited planets were invaded by the Zerg and destroyed by his fleet, he hesitated and pulled out of the Terran Confederacy to examine the situation further.
After the first world the Zerg took from the Humans 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 scour 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, Humans 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 Humans 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 Humans 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.