After leaving the Protoss Homeworld of Aiur the Xel'Naga Worldships traveled thousands of light years away into the burning core of the galaxy. They took their gigantic Worldships into orbit over the volatile ash-world known as Zerus. There they set about to continue their Grand Experiment. They labored to advance the the most insignificant life form on Zerus, the insectoids called Zerg. Over a suprisingly short amount of time the strains grew to resemble a terrifying ravenous and unified race. The Xel'Naga remembering their failure with the Protoss constructed a huge brain-like entity called the Overmind to control the primary drives and instincts of all the Zerg strains. This Overmind then used secondary agents, Cerebrates to carry out various objectives of the Overmind. The Queens were used to oversee drone activity, and finally Overlords were used to relay orders in battle and transport Zerg warriors into the fray. The Overmind realized that to further evolve they would need to leave this planet and find others. Gargantuan, space-faring lifeforms came to Zerus and were quickly assimilated. The Overmind broke the psionic link with the Xel'Naga and launched the space-faring Zerg swarms at the unsuspecting Xel'Naga Worldships. Within only a few hours the Zerg overran the defenses of their creators and laid waste to the Xel'Naga fleet. The Zerg learned of the Protoss and began the journey to Aiur laying waste to every planet along the way. Needing a way to combat the psionic ability of the Protoss the Zerg found 'Humanity' and went to harness their psionic power. So the infestation of Chau Sara began.
