"I bring tidings of doom. The xel'naga return, the cycle nears its end, the artifacts are the key... to the end of all things."

- Zeratul delivers a warning to Jim Raynor

The xel'naga are a psionically-attuned, ancient, cosmic race. They were occasionally referred to as the most powerful species known in the universe. While there are vague references of the xel'naga conquering worlds and uplifting species, their most notable involvement was with the protoss and zerg. Their interest in these two species was due to the xel'naga's cyclical life cycle. When the incredibly long-lived xel'naga were due to come to an end, the xel'naga would use these two species to be reborn. Ironically their survival was severely jeopardized when the zerg turned on them.

The xel'naga cherished all life and refused to kill anything, not even the entity at KL-2. The protoss eventually followed a version of this philosophy, called the "Dae'Uhl".

The terminology of the word "xel'naga" has been described as a protoss language term meaning "Wanderers from Afar" but has also been described as their true name. The protoss referred to the xel'naga as the Ihan-rii, the "Great Teachers", the "Makers", and the "Guardians",[5] as well as "gods".
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The xel'naga were incredibly long-lived by protoss standards, but 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 the purity of essence and the purity of form, and each uplifted species would be targeted for one of them. Over an extraordinary long period of time, the two species would naturally come together and merge.
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The Cycle

The xel'naga life cycle had manifested many times.

The xel'naga arrived in the Milky Way galaxy from outside. Tens of millions of years before the Great War, the xel'naga seeded thousands of new species in the worlds within their domain. They experimented on and uplifted suitable candidate species. For millenia they met only with failure.

The following section contains timeline information which isn't necessarily in chronological order.

During their journeys, they learned of the Voice in the Darkness, a Void-based entity which consumed entire civilizations. The xel'naga, who cherished life, imprisoned it with an Argus crystal beneath a barren world at KL-2.

The xel'naga found or created a powerful relic, the Argus Stone, which they valued over other artifacts, but left it on the world of Aridas. The zerg believe it had been used to create entire races.

The First Born

Protoss history

On the galactic fringe the xel'naga returned to Aiur, one of their engineered worlds. They discovered there a race of tribal hunters with a psionically-enhanced communal society. When coupled with excellent physical attributes the xel'naga recognized the natives as superior candidates for the purity of form and, impatient with the slow pace of their evolution, began uplifting them, using devices such as a giant crystal beneath Aiur. They even used devices such as the Kassia crystal to enhance their rate of reproduction. The natives were the first experiments to progress beyond a feral stage. In recognition of this the xel'naga named their new scions protoss, meaning first born.

Initially the xel'naga kept their presence and assistance secret, but once the protoss were deemed ready the xel'naga revealed themselves openly. A "protoss" temple was built where the xel'naga first landed on Aiur. The xel'naga also constructed giant artificial caverns or a city beneath Aiur, and hid khaydarin crystals around the planet.

The protoss were united by their fascination with these new "gods." They called their new benefactors the Ihan-rii, the "Great Teachers", the "Makers", and the "Guardians". The Shelak Tribe grew closest to the xel'naga. The protoss progressed rapidly technologically but rifts began appearing between the tribes concerning their perception of the xel'naga. At first, the xel'naga were intrigued by this ideological development, and conducted an experiment to see what effects the diverging tribes might have on khaydarin crystals. They created twin crystals, the Khalis and Uraj, from a monolithic crystal and gifted them to the Sargas and Akilae Tribe respectively. The crystals' powers and appearance diverged with their tribes.

The ideological split proved more severe than the xel'naga realized. The protoss fractured along tribal lines and caused the psionic communal link to break. The xel'naga were disappointed but realized having pushed the protoss too far too fast they were responsible. The xel'naga decided to leave this creation. The xel'naga were already done with the protoss, and needed to work on a new species.

Hundreds of xel'naga were killed in attacks by angered and confused protoss. While their fleet withdrew, the xel'naga left behind many artifacts such as the khaydarin crystals. At least one ship was also left behind.

The protoss degenerated into the Aeon of Strife. However the xel'naga's mark on the protoss endured, and their abandoned artifacts later aided the protoss to re-emerge from the chaos as a galactic power.

Broken Cycle

Zerg history

While the protoss were regressing they had all the essential qualities of the purity of form. The xel'naga set off to find a suitable companion embodying the purity of essence. They found a suitable candidate on the coreward world of Zerus. There the xel'naga began genetically manipulating the zerg, an insignificant life form of miniature insectoids. The initial experiment was a resounding success. The parasitic zerg quickly became the dominant lifeform on the planet, with other creatures being assimilated and evolved or simply eliminated. The xel'naga were shocked to discover how quickly assimilated species could be changed beyond recognition within the space of a few generations.

In the view of the xel'naga, the protoss had failed due to the independence of the individual and conflicts of ego that arose from that. To avoid the same with the zerg, the xel'naga consolidated control of the collective zerg sentience into a single entity, the Overmind. The xel'naga used a psionic link with the Overmind to monitor the zerg, but the xel'naga kept the link secret as to not reveal their presence.

However, it was not long until the Overmind realized that to continue growing the zerg would need to leave Zerus, and assimilated the behemoths, enabling its forces to travel into space. It had since detected the xel'naga worldships in orbit and these made a logical first target for new material. The first sign that the xel'naga had that something was wrong was when the Overmind severed the psychic link. The zerg launched an overwhelming attack on the worldships. The "greater whole" of the xel'naga were slain, an action which threw the xel'naga plan into turmoil.

The Overmind assimilated some of the xel'naga's knowledge as it processed thousands of xel'naga into itself. Amongst other things, the zerg learned of the protoss, the other great experiment; they would become the zerg's ultimate target for assimilation.

Return of the Xel'Naga

Shortly before the Brood War, the xel'naga temple on Bhekar Ro was unearthed. The temple housed a strange "phoenix-like" energy creature which absorbed the essences and genetics of both protoss and zerg.

At the end of the Brood War, Zeratul stumbled upon a shocking secret on a dark moon, where he discovered a protoss/zerg hybridization project overseen by the mysterious Samir Duran, who had previously appeared as an infested terran. Duran told Zeratul that Kerrigan was not in charge of his experiment, "although her rebirth into the Zerg Swarm has sped up my progress." Duran also told him that he was "a servant of a far greater power" and "this creature is the completion of a cycle. Its role in the cosmic order was preordained when the stars were young."

Samir Duran's pronouncements on the hybrids being the "completion of a cycle" led Zeratul to seek out information on this new threat. However, he was suffering mentally for numerous reasons, such as the death of Matriarch Raszagal at his hand. Years after his discovery, Zeratul was located by a preserver, Zamara, and her terran "host", Jake Ramsey. They restored Zeratul's mood and exchanged information. According to Zamara, Samir Duran's experiments are a "perversion", and if his attempts to "unify" the species become successful, "a monstrous and powerful perversion of both protoss and zerg will be set loose upon the universe, and all that [the protoss and terran] know and cherish will fall in their wake".

Zeratul traveled to the world of Pegasus, where a xel'naga temple hatched. He followed the energy creature through a wormhole to a mysterious planet on the other side.

Wings of Liberty

"Do you hear them Zeratul? Whispering from the stars? The galaxy will burn with their coming."

- Kerrigan to Zeratul, possibly referring to the xel'naga

On a lost world, Zeratul found evidence of a prophecy of the xel'naga's return. But the question remained if they come to save, or destroy.

Zeratul appeared on the Hyperion, command ship of Raynor's Raiders. Jim Raynor had been acquiring alien artifacts to sell to the Möebius Foundation. Zeratul told him that "the xel'naga return" and that the artifacts Raynor has acquired hold the key to the end of the cycle and the end of all things.