The Overmind is a bodiless 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 Zerg larvae. Direct orders were relayed through Queens and 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 Overmind travelled with its Swarm to the vicinity of Aiur. It subtly infected the Terran world of Chau Sara with Hive Spores as a prelude to invasion. It hoped to use the Terran psionic potential to help it defeat and possibly assimilate the Protoss.
Later, it nestled onto the planet Char, whose remote location and convoluted terrain would protect it from its enemies.
The Overmind's plan was partially successful - it acquired the services of Infested Kerrigan, a powerful minion with augmented psionic powers. Unfortunately for it, she drew the attention of Zeratul and the Dark Templar to Char.
The Dark Templar leader, Zeratul, and Tassadar conspired to perform a "weapons test" against the Zerg. They had discovered the purpose of Zerg Cerebrates and believed that 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. Using the power of the Khaydarin Crystal and the cosmic nexus found underneath a Protoss Temple, which marked the spot where the Xel'Naga first set foot on Aiur, the Overmind was able to physically manifest itself on Aiur. It was protected by a powerful defensive shell. Despite successes against the Protoss defenders, it was unable to assimilate the species.
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.
In the final confrontation with the Overmind, the Protoss drew upon the services of the Dark Templar along with Raynor's Raiders. Together, both forces fought their way to the Overmind. The Overmind tried to escape through the use of a warpspace rift, but Tassadar was able to destroy it, along with himself, through the use of Dark Templar energy before it could escape. He rammed his carrier, the Gantrithor, directly into the Overmind.
The Second Overmind
By Episode IV, it was revealed that the Zerg had formed a new, young Overmind on Char, the adopted Zerg homeworld, when Senior Cerebrate Daggoth ordered a number of other Cerebrates to fuse, but it was captured by the United Earth Directorate in Episode V.
In Episode VI, the final act of the PC games, Infested Kerrigan, the self-proclaimed Queen of Blades —a former human Ghost telepath soldier who had been infested (transformed) into a Zerg/Human Hybrid— had the second Overmind killed and claimed leadership over the entire Zerg Swarm. Because of Kerrigan's psionic prowess and ability to control every Zerg, no new Overmind has arisen. Kerrigan can be considered ruling in a similar way as the Overmind at this point, because she serves all the functions the original Overmind itself did.
Powers of the Overmind
It is biologically impossible for the Zerg to oppose its directives. Although the Overmind is a living being, it is really an entity that embodies the whole will of the Zerg species. Because of this, the Overmind is an immortal being; it can never really be killed except through the use of Dark Templar energy.
The Overmind is able to create rifts into warpspace to transport its minions at faster-than-light speeds. It used these rifts at least three times during the course of the StarCraft game.