Ouros

Ouros was a xel'naga.

Biography
Ouros was freed from confinement in the Void by a joint Daelaam, Dominion, and zerg force in the final stages of the End War. This was allowed through the final defeat of Emil Narud, his jailer. After being freed from Narud's grasp by the allied forces, Ouros explained that it was he who had been guiding both Zeratul and Artanis with visions and warnings, and not the spirit of their deceased freind and mentor Tassadar, whose form Ouros had subverted and used to gain the trust of both Protoss to better impart his wisdom and warnings to them.

He revealed that only another Xel'naga could hope to defeat Amon utterly while he resided in the Void, as it was 'an extension of his will' and would do everything he commanded. 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 she would need to defeat Amon.

Over the vehement objections and pleas of Jim Raynor, who had no wish to see Kerrigan turned into one of them, she agreed to do it. Kerrigan insisted that it was the only way not just to defeat Amon, but to redeem her own soul for the billions of lives she had taken during her revenge against Arcturus Mengsk almost a year ago.

Thus, Ouros transferred his essence into Kerrigan while Raynor, Artanis and Broodmother Za'gara (leading the Zerg Swarm) provided protection against the waves of void entities and Void Thrashers Amon send against them to stop the ritual.

Eventually however, the transfer was successful, as the last of his essence was transferred to Kerrigan to allow her to ascend and complete the ritual, Ouros spoke of how the infinite cycles were now coming to their end.

It is not known what became of him after Kerrigan's ascension, it is likely he sacrificed himself to merge with her.