Era of CreationEdit
Ouros was one of the xel'naga who helped construct the material universe and seed it with life to perpetuate the Infinite Cycle. At some point, he was captured by Amon while the rest of his race was extinguished. He was chained in the Void, which was twisted to the will of Amon. However, he could still subtly influence the material universe, leaving clues to help guide his creations to rescue him.
Prophecies and VisionsEdit
Ouros knew of Amon's plan to return to and destroy the material universe, and sought to make it aware of his plans as well as how to stop them. He imparted on the protoss race the Ulaan Prophecy, which told of the coming of the zerg and their Overmind, and that it heralded the coming of the Fallen One. The Overmind too was given a vision of a future where Amon dominates its swarm and uses it to destroy all life in the universe.
After the Brood War, Dark Prelate Zeratul sought the guidance of the ancient prophecies to understand the plans of Samir Duran, a xel'naga plotting to bring Amon to the material universe who was breeding the hybrid. He came across Ouros's prophecy, and was guided to the area where the Overmind fell. There, Ouros was able to contact him personally in the form of a ghostly apparition of Tassadar in order to have Zeratul trust him. He gave Zeratul the vision of the future the Overmind saw, and informed him Sarah Kerrigan had to live in order to prevent this future.
Zeratul then figured from Ouros that Kerrigan needed to be remade on the planet Zerus, replacing her corrupted zerg essence with that of the primal zerg. Zeratul stated it was the will of the xel'naga for this to occur, implying this was also a vision given to him by Ouros. This would form her into a suitable host to perpetuate the Infinite Cycle.
This done, Zeratul needed to fulfill only one more piece of the prophecy; that in the land where Amon would be reborn, the last light would be made clear. Zeratul tracked this location to the Temple of Erris on Atrias. Zeratul pushed through the Tal'darim and hybrid in the temple, destroying the void catalyst that allowed them to communicate with Amon. The unleashing of void energies allowed Ouros to once again communicate to Zeratul using the voice of Tassadar, stating that the Keystone would usher him unto hope.
This would set off the chain of events that would lead Hierarch Artanis and Kerrigan to Ulnar, which served a resting place for the xel'naga and a portal to the void. Though the xel'naga were dead, the Keystone also gave Artanis the weapon he needed to rip Amon from the material universe and force him back into the Void.
Into the VoidEdit
After Amon was thrown back into the Void, Ouros called to Kerrigan, who gathered up Artanis and Commander Jim Raynor. She stated his voice was benevolent, and that they needed to destroy Amon in the Void before he came back. The three fleets followed the voice, who once again came to them as Tassadar. He was being jailed by the Duran, who had been cast into the Void by Kerrigan. After slaying him, they freed Ouros from his prison.
After being freed from Duran's grasp, Ouros explained that it was he who had been guiding both Zeratul and Artanis with visions and warnings. He revealed that only another xel'naga could hope to defeat Amon utterly while he resided in the Void, as the entire dimension was 'an extension of his will.' Ouros implored Kerrigan to embrace her destiny and part in the prophecy by merging with his essence and ascend to become a new xel'naga, as she was the only one capable of holding the immense energy and power needed to defeat Amon and perpetuate the Infinite Cycle.
Ouros transferred his essence into Kerrigan while Raynor, Artanis and Broodmother Zagara provided protection against the waves of shadows of the Void and void thrashers that were sent to destroy her. Eventually, the essence transfer was complete. Ouros closed his eyes and faded from existence, stating that the infinite cycles had finally come to their end. Kerrigan would use the power he had given her to destroy Amon, ending him as a threat to the universe.
The name Ouros bears resemblance to Ouroboros, symbolized by a snake eating its own tail. In mythology it is a symbol of cyclicality, which is likely a parallel to the Infinite Cycle.