Believing that their mistake with the Protoss was pushing the sentience of the young race too quickly and allowing the differing egos to clash with one another, the Xel’Naga set out on a different path in creating the Zerg. They structured this new race in such a way that all of their collective sentience was gathered in one collective intelligence, or “Overmind”. The original Overmind was nothing more than a semi-sentient being that represented the primary drives and instincts of all the Zerg strains.
Over time however, the Overmind evolved and began to show signs of personality and advanced thinking. Along with its new mental capacities the Overmind found that it was immortal, for how can one destroy the sentience of billions of creatures? If the Overmind's physical form were to be struck down, it would simply be reborn in another Zerg hive and resume its activity. The Overmind controls every creature within the swarm but it does not do so through direct interaction, it instead gives orders to “Cerebrates” who in turn give orders to Zerg Overlords, who finally give the orders to individual Zerg creatures.
As the Overmind grew more and more complex it started to think about its own future. It saw that within just a few centuries of its creation the Zerg swarm had assimilated all of the indigenous life on their homeworld of Zerus. If the Swarm was going to continue to grow it would need to leave their desolate homeworld. The means for that journey soon presented itself in the form of huge spare-faring life forms. The Overmind called out to them using its immense psyche and when the creature came to investigate the signal they were quickly assimilated into the swarm.
Now with means to leave Zerus the Overmind set its sights on one final goal before the swarm’s departure. The Xel'Naga had continued to monitor the Zerg over the centuries and were very happy with the results. Not only did the Zerg show an amazing propensity for evolution, but thanks to the Overmind kept the purity of their essence. However, they were too busy congratulating themselves to see that the Overmind had severed the psychic link that they had established to keep control of it. The Xel’Naga were caught unaware by the sudden Zerg onslaught and the great majority of the race was slaughtered or assimilated.
With both the strength to travel the great void of space and near limitless knowledge of the Xel’Naga, the Overmind was more powerful that it had ever been before. With this knowledge it saw that in the Zerg the Xel’naga had created the purity of essence, and in the Protoss the purity of form. The Overmind now knew what its existence was for and set off into space with its swarm in search of Auir and the Protoss, to create the perfect life-form.
However, some time after the creation of the Overmind, a vengeful surviving Xel'Naga managed to reactivate the long-dormant psychic command which forced it to obey the orders of its creators. This Xel'Naga, the Fallen One, ordered the Overmind to continue with its plan to create the ultimate lifeform. Using its immense intellect, the Overmind extrapolated future events and realized that the Fallen One planned to use the Swarm and the Zerg-Protoss Hybrids to wipe out Humanity, the Protoss, and all other life-forms in the galaxy, then exterminate the Zerg when its dark task was complete. Despite this knowledge, the Overmind literally could not disobey the orders of the Fallen One, so it came up with an alternate solution. It captured Terran Ghost Sarah Kerrigan and transformed her into the Queen of Blades, a powerful Zerg-Human hybrid who, not being originally created by the Swarm, was not shackled with the psychic link to the Fallen One. Then, during its campaign against Aiur, the Protoss homeworld, it constructed its physical form on the surface of the planet for the first time in centuries. This was a deliberate act of suicide; it knew that the incredibly powerful psychic powers of the Protoss would give them the ability to actually kill the Overmind once and for all, and that when it was dead, Kerrigan would be powerful enough to take control of the Swarm herself. In allowing itself to be destroyed and the will of the Swarm granted to Kerrigan instead, the Overmind ensured the survival of the Zerg species, and indirectly, created the greatest hope for the Terrans and the Protoss as well.