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The Infinite Cycle was a cycle of rebirth enacted by the xel'naga.
The xel'naga arose within the Void. While incredibly long lived by even protoss standards, the xel'naga were not immortal. Eventually, their species would come to an end. In the interest of survival they planned to create new xel'naga by uplifting two species. A pair of qualities was required and these were defined by the xel'naga as purity of essence and purity of form, and each uplifted species would be targeted for one of them. Over an extraordinarily long period of time, the two species were believed to naturally come together, forming the next generation of xel'naga and beginning the cycle anew. Pure of essence and pure of form would appear together before the xel'naga at Ulnar, and use the Keystone to awaken the xel'naga who would then give their essence to create the new generation of xel'naga.
The cycle was repeated in numerous universes: at the start of a universe's creation, the xel'naga would take on physical form and enter the universe, seeding it with life. The cycle repeated so many times that time itself was affected by it. Each time, the xel'naga observed and identified the potential of their offspring, but never interfered directly.
In what would be the last universe to bear witness to the cycle, the xel'naga seeded the universe with life through Ulnar. They entered slumber, waiting for the two destined races to find and assemble the Keystone, which would lead them to Ulnar, beginning the cycle anew. Unfortunately, one of their brethren, Amon, sought to break the cycle as he saw it as corrupt and desired to remake the universe in his image. Gathering like-minded followers, he departed into the universe and, breaking his kind's laws, directly interfered with the evolution two races, the protoss and zerg, and instilled into them purity of form and purity of essence respectively. He intended to forcibly merge the two species and create the hybrid. His brethren eventually discovered his plans and waged war against him. In the ensuing War Among the Gods, Amon unleashed the zerg upon the xel'naga, killing most of them, but Amon himself was defeated and banished back into the Void. The surviving xel'naga returned to Ulnar to slumber. However, a survivor of Amon's forces, Samir Duran, would continue his master's work in secret.
Shape of Things to Come
As thousands of years passed, some protoss preservers would become aware of the knowledge of the xel'naga's Infinite Cycle, but kept it as hidden knowledge. As events unraveled between the protoss and the zerg and the role of the xel'naga became more apparent in their conflict, the dark archon Ulrezaj sent assassins to kill off the preservers to destroy their knowledge of the Infinite Cycle for some unknown end. While most were slain, one named Zamara managed to survive long enough to give this knowledge to the protoss and some terrans. This knowledge became invaluable to Dark Prelate Zeratul, who continued to investigate the schemes of Samir Duran.
The Last Turning of the Cycle
- Ouros's last words(src)
Duran ensured the zerg and protoss would come into conflict, allowing him to start creation of a false xel'naga army for his master known as the hybrid. Duran also implanted himself in the Moebius Foundation, manipulating events so he could use the xel'naga Keystone to absorb enough energy from the Queen of Blades to pull Amon from the Void. Duran's plan was successful, and Amon returned. He moved on Ulnar and slew the slumbering xel'naga within. However, Dark Prelate Zeratul was forewarned by the Ouros, a surviving xel'naga trapped in the Void, and ensured Kerrigan survived and was cleansed of the corruption Amon imparted in the Zerg Swarm early in their uplifting. Kerrigan banished Duran back into the Void and disrupted his hybrid birthing operation, but Amon rose again and inhabited the protoss Khala.
Using his hybrids, terrans of Moebius Corps subjugated to be his thralls, feral zerg of the Amon Brood brought under his control, the Tal'darim, and the corrupted Khalai protoss, Amon waged war on the galaxy, with the intent of exterminating the xel'naga's creation. In doing so, the xel'naga could not inhabit a new form, and the cycle would be broken. However, Amon's rage was great, and he did not just want the creations of the broken cycle to be destroyed, but to suffer. Hierarch Artanis of the Daelaam attempted to reawaken the xel'naga in Ulnar to stop Amon, but found them slain, and Amon taunted him that he was constructing a hybrid host body on Aiur. Artanis united the Nerazim, Purifiers, and Tal'darim, and moved on Aiur, destroying Amon's host body and using the Keystone to pull him from the Khala, banishing the fallen xel'naga back into the Void.
Amon's conquest to destroy the cycle had been halted, but not forever, as he still existed within the Void, and he immediately began plotting to return to the material universe. Sarah Kerrigan united the Terran Dominion forces under Commander Jim Raynor and the Daelaam under Artanis, and sent a force into the Void to face him. There, they fought Duran, who was keeping Ouros imprisoned. Alexei Stukov slew Duran for the final time, freeing the xel'naga. Ouros insisted that Kerrigan had to ascend to a xel'naga, and he would impart his essence to her. Kerrigan agreed, and in spite of Amon's attempted intervention the cycle turned over one last time, and Kerrigan became a xel'naga. Kerrigan and her allies personally fought Amon, and disarmed the shield surrounding him. Amon insisted she was the creation of a flawed cycle, but Kerrigan insisted that she didn't care for the Infinite Cycle or the plans of the xel'naga, and that all she cared about was freedom for all. With one final blast, Kerrigan slew Amon, ending Amon's plans to exterminate all products of the cycle.
The Legacy of the Void mission "The Infinite Cycle" seemingly implies (though never explicitly states) that two separate species, one of purity of essence, the other of purity of form, assemble the Keystone and arrive at Ulnar where they are merged into a single race by the elder xel'naga themselves. The Dark Templar Saga novels, however, state that this merger of essence and form is meant to occur naturally over time - indicating that the resulting hybrid race is meant to assemble the Keystone and find Ulnar (thereby, in a sense, bringing "both" races to Ulnar).