The Zerg Overmind — or simply the Overmind — and after the Brood War also known as the first Overmind, was the leader of the Zerg Swarm. It was created on the Zerg homeworld of Zerus by the Xel'naga in order to bring order to the zerg.
The Overmind was a brain-like entity of indeterminate age. It was created by the Xel'naga on the Zerg homeworld of Zerus to bring order to the zerg. At first it was only a semi-sentient entity created from the instincts and collective sentience of the Zerg, but it quickly evolved. The Overmind did not directly relay orders to its minions. As new species were added to the Swarm, it began to relay orders telepathically through cerebrates, giant versions of the larvae. Direct orders were relayed through overlords.
As the Overmind grew stronger, it extended its senses to the space over Zerus, detecting the Xel'naga worldships which floated above. Cutting off its psionic link to the Xel'naga, it prevented them from knowing what it was doing. It called a naturally space-faring species from the void of space to the planet and assimilated it, enabling its minions to survive in space. Then it commenced its attack. The Overmind absorbed thousands of sentient Xel'naga into itself, gaining their knowledge and insights and causing it to grow much more powerful. It learned the secrets of the sacred khaydarin crystals, and began to incorporate the energies of these crystals into its own. Through the knowledge 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.
The Overmind dissected the memories of the Xel'naga experiments with the protoss. The Zerg then devoted their energies to seeking out the powerful Protoss in the hope of absorbing them into the Swarm, uniting purity of form and of essence, to create a totally "perfect" being.
Conflict with the Protoss
The swarms traveled in the direction of the Protoss homeworld. For thousands of years, the Zerg wreaked havoc upon the galaxy, destroying or assimilating countless of species it encountered and laid hundreds of planets into smoking ruin. As the swarm added ever new strains to its formidable army, the Overmind was still not pleased. It sought a way to counter the awesome might of the protoss psionics, but found no answer among the strains it devoured.
All changed when one of its deep-space probes encountered humanity, a frail race that lived under the very shadow of the Protoss. Humanity had a great psionic potential and the Overmind ordered the assimilation of the Terran worlds to gain the ability to fight the Protoss on its own terms. It took the Overmind sixty years to reach the series of worlds inhabited by the Terrans, sending out a scouting brood to determine the locations of their worlds.
The Overmind subtly infected the Terran world of Chau Sara with spores as a prelude to invasion. The Zerg infested or killed most of the populace and had sent minions to other Terran worlds by the time the Protoss discovered what had happened and responded. The Overmind, anxious to learn all it could about the Protoss, held its minions back and let the Protoss hamper the initial infestation process. The Protoss destroyed all life on the surface of Chau Sara. The decisive action of the Protoss pleased the Overmind, which marveled at how the Protoss could bring about such devastation. The Overmind pulled its forces back to see how humanity and the Protoss would react next.
The Terran terrorist leader Arcturus Mengsk sought to use the Zerg as a weapon of war against his enemies, the Terran Confederacy. To this end he acquired psi emitters, a device which amplifies the powers of a telepath, enabling them to attract Zerg from a considerable distance.
Agent of the Swarm
The Zerg had not yet reached Tarsonis, the capital world of the Confederacy, when they were drawn there by the emitters. The Protoss arrived, intending to destroy all life on Tarsonis, but Arcturus Mengsk had his own plans. His Sons of Korhal, under the leadership of the increasingly politically unreliable ghost Sarah Kerrigan, would attack the Protoss, preventing them from destroying the Zerg. In turn, the Zerg would destroy the Confederacy. The plan was successful, but Mengsk abandoned Kerrigan to the Zerg. Kerrigan was not killed by the Zerg, only captured and infested.
The Overmind created a new cerebrate with only one purpose in mind; to protect the chrysalis which now held Kerrigan. The Overmind told the cerebrate that as long as the chrysalis and the creature inside survived, the cerebrate would also continue to live. The Overmind moved the chrysalis and the rest of the Swarm to the planet Char, whose remote location and convoluted terrain would protect it from its enemies.
Infested Kerrigan emerged from her cocoon during an attack by Raynor's Raiders. The Overmind referred to her as "the undoing" of its enemies and ordered the deaths of all the invading Terrans. However, Kerrigan allowed Raynor to leave with his life.
Kerrigan's mind had been damaged by the Ghost Program during her training, and she wished to undo the damage and access her entire range of powers. She intended to raid a Terran Dominion science vessel to learn how to undo this damage. Cerebrate Zasz took issue with this plan, however, reminding her that she was just a servant, and that her wishes did not take precedence over the Overmind's mission. Kerrigan reacted badly to this pronouncement, but the Overmind allowed her to carry out her desire – while she had retained most of her spirit, she could not stray from the Overmind's will.
A New Weapon
Once she had expanded her mind, Infested Kerrigan could detect the Protoss who had been hiding on Char. The Dark Templar leader, Zeratul, and High Templar Tassadar conspired to perform a "weapons test" against the Zerg. They had discovered the purpose of the cerebrates and believed that only Dark Templar energy could permanently destroy one. Tassadar distracted Infested Kerrigan while Zeratul slew Cerebrate Zasz of the Garm Brood. This created a psionic link between the Overmind and Zeratul. Each learned the other's secrets. The shock of permanently losing a cerebrate threw the Overmind into a state of mental shock. It did not emerge until the remnants of the Garm Brood had been destroyed. Senior Cerebrate Daggoth commanded the Zerg Swarm while the Overmind was unable to communicate.
The Overmind changed its plans. It would travel to Aiur, whose precise location it had drawn from Zeratul's memory, but leave Infested Kerrigan behind on Char in order to fight the Protoss remaining there.
The Overmind ordered its forces to clear the "hallowed ground" upon which stood the original Protoss temple on Aiur (the site of where the Xel'naga first set foot on Aiur) and place the khaydarin crystal in its place. Only there could it be made manifest. The crystal was placed, after which the Overmind dived upon Aiur like a bomb and implanted itself on the surface.
Infested Kerrigan had some success in hunting down the Dark Templar, capturing Zeratul and many others in a Terran installation. However, Tassadar was able to recruit a Protoss task force and rescue them. These Dark Templar were transported to Aiur. After winning a civil war with the Protoss Conclave, Zeratul and his allies (including Tassadar, Fenix and the Terran, Jim Raynor) plotted their attack on the Zerg. Fenix and Zeratul intended to permanently destroy a pair of cerebrates, which would thin the Overmind's defenses and distract the Overmind. The attacks were successful, opening the path to the Overmind. In the final confrontation, the Protoss drew upon the services of the Dark Templar along with Raynor's Raiders. Together, both forces fought their way to the Overmind and destroyed its protective shell. The Overmind attempted to escape through a massive warp rift it had created above itself but it was too late. Tassadar channeled both the High and Dark energies into a gigantic glowing ball of energy and destroyed the Overmind, along with himself. The Overmind's shattered remains were sucked into the warp rift above.