Following the perceived failure of the Protoss, the xelnaga set off once more to create the perfect lifeform. They discovered Zerus and the zerg, the most insignificant lifeform upon the planet. Using the zerg as the base the xel'naga continued their grand experiment. The development of the zerg reached its climax when the nascent Overmind broke away from the xel'naga's control and launched an attack on its progenitors. Having absorbed a portion of the xel'naga's knowledge, the zerg ascended to the stars to seek their destiny and the protoss. Zerus was left a lifeless rock.