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. It resulted in massive casualties on both sides, and the assimilation of the xel'naga's knowledge by the Overmind.
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. They moved to the world of Zerus, where they discovered the native creatures - the zerg - were capable of stealing essence. 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. 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.  He forced upon the Overmind an overriding directive to seek out and forcibly assimilate the protoss. Although the Overmind understood that Amon's schemes would ultimately result in the extinction of the zerg, it could not disobey its primary directive.
However, around the time the Khala was created on Aiur, the xel'naga in Ulnar awoke. Feeling the absence of Amon and his followers in the Void, they wandered out and discovered Amon and his followers' experiments at Zerus. Appalled by what their brother had done, the xel'naga moved their worldships to stop Amon's plans.
A brutal war began in the skies over Zerus, and Amon released his final weapon against his kin: the zerg swarm. Although the xel'naga had initially been monitoring the Overmind through a psychic link, the Overmind severed this link, enabling it to launch a surprise attack on the xel'naga fleets in orbit. Within mere hours, the Swarm overran the xel'naga's defenses, destroying many of their worldships, and killing and consuming most of the xel'naga. 
Despite suffering severe losses at the hands of the zerg, the surviving Ulnar xel'naga defeated Amon and his followers in the final battle. Amon himself was banished back into the Void. Although his physical body was destroyed, Amon took control of the Void and began plotting his return. The few surviving xel'naga loyal to the Infinite Cycle returned to Ulnar to slumber. 
In the aftermath, the Overmind assimilated the knowledge from the thousands of xel'naga the Swarm had devoured, allowing it to grow more powerful than it ever imagined.  However, it was unable to escape Amon's directive to destroy and assimilate the protoss. The Overmind and the Swarm subsequently left Zerus and sought out the protoss, though they did not know the precise location of Aiur.  At the same time, the Overmind secretly sought a way to free the zerg from Amon's control.  Meanwhile, a surviving xel'naga loyal to Amon, Samir Duran, schemed to use the conflict between the zerg and the protoss to revive his banished master, as well as to create a body to sustain him.
- ↑ 1.0 1.1 1.2 1.3 1.4 Underwood, Peter, Bill Roper, Chris Metzen and Jeffrey Vaughn. StarCraft (Manual). Irvine, Calif.: Blizzard Entertainment, 1998.
- ↑ Blizzard Entertainment. StarCraft II: Heart of the Swarm. (Activision Blizzard). PC. Zurvan (in English). 2013.
- ↑ 3.0 3.1 3.2 Blizzard Entertainment. StarCraft II: Wings of Liberty. (Activision Blizzard). PC. Mission: Wings of Liberty, Echoes of the Future (in English). 2010-07-27.
- ↑ 4.0 4.1 4.2 4.3 Blizzard Entertainment. StarCraft II: Legacy of the Void. (Activision Blizzard). PC. Rohana (in English). 2015.