Appalled by what their brother had done, the xel'naga moved their worldships to stop Amon's plans.
Feeling the absence of Amon and his followers in the Void, they wandered out and discovered Amon and his followers experiments at Zerus. However, around the time the Khala was created on Aiur, the xel'naga in Ulnar awoke. Although the Overmind understood that Amon's schemes would ultimately result in the extinction of the zerg, it could not disobey its primary directive. He forced upon the Overmind an overriding directive to seek out and forcibly assimilate the protoss. To ensure the zerg did not fall to the same infighting as the protoss had, Amon created a physical embodiment of the hive mind: the Overmind. They subsequently uplifted the zerg, but when it became clear that the zerg were independent and would not follow, Amon decided to bind them to a hive mind.
They moved to the world of Zerus, where they discovered the native creatures - the zerg - were capable of stealing essence. Having failed with the protoss, Amon and his followers decided to focus on uplifting a species with purity of essence in order to create false xel'naga to help them end the Infinite Cycle. It resulted in massive casualties on both sides, and the assimilation of the xel'naga's knowledge by the Overmind. The War Among the Gods was a conflict that occurred between the xel'naga loyal to Amon and the xel'naga who stood for the Infinite Cycle.