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.
The etymology 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", as well as "gods".
Xel'naga biology varied based on the creature. Many, such as Ouros, Amon, and those depicted in the xel'naga temples showed a large squid-like creature with a bulbous body. Others such as Narud were large hunched creatures, similar to void thrashers. Sarah Kerrigan's xel'naga form was that of a flaming human with large wings. 
Creatures containing xel'naga essence echo numbers of the Cuccodujo series, as opposed to terran flora and fauna which echo the Fibonacci series.
The xel'naga, and creatures that contain xel'naga essence, are able to passively facilitate the rapid growth of life around them, and cause extreme growths of flora and fauna even in dead worlds.
The xel'naga were incredibly long-lived even 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 extraordinarily long period of time, the two species would naturally come together and merge.
The xel'naga life cycle had manifested many times.
Arrival in the Milky WayEdit
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 millennia they met only with failure.
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.
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.
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.
The Overmind's free will was undermined by an overriding directive to destroy the protoss. This was part of a greater plan by the fallen xel'naga Amon to remould creation in his own image. The Overmind disagreed with its directive but could do little to resist.
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.
Meanwhile, at least one worldship survived in the Sigma Quadrant.
Shortly before the Template:Link, 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 protoss/zerg hybridization project overseen by Samir Duran; the project was for "a far greater power" to which Duran ultimately worked for. Thought not Kerrigan's project, her "rebirth into the Zerg Swarm" had expedited progress. Furthermore, Duran claimed the hybrid was the "completion of a cycle", and its existence was "preordained when the stars were young."
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 LibertyEdit
On the lost world of Ulaan lay three prophetic fragments contained within shrines. The zerg Swarm under the Queen of Blades had traveled there and occupied the area around them. Zeratul himself arrived, only to encounter the Queen of Blades. They clashed over what the future beheld. Kerrigan had already given up, saying their conflict meant nothing. Zeratul left shortly after Kerrigan, believing there was still hope. Zeratul then located the prophecies with help from Praetor Karass and his forces. Kerrigan attacked the protoss, and at Karass's urging, Zeratul escaped from Ulaan vowing Karass's sacrifice would not be in vain.
Zeratul traveled to the world of Zhakul, hoping to make contact with a trio of "immortal" preservers there to interpret the prophecy. However, a hybrid, Maar, serving a higher power, had taken control of the Zhakul Guardians and was using the preservers as a source of energy. It stated "some SECRETS must remain HIDDEN" as it assisted the Zhakul Guardians in fighting off Zeratul's Enclave. However, Zeratul's forces defeated the Zhakul Guardians, killing Maar and freeing the preservers.
After hearing the preservers' interpretation, Zeratul believed the prophecy referred to the Overmind and resolved to get information from it. On Aiur, at the site of the Overmind's demise, Zeratul encountered the spirit of Tassadar. The spirit revealed the Overmind had been corrupted and enslaved, given an overwhelming directive. However, it created Infested Kerrigan as a free-willed leader in an effort to escape its destiny. Tassadar imparted a vision of a possible future, in which a mysterious being, the Dark Voice known to the Overmind as the "Fallen One", used the hybrids to take control of the Zerg Swarm in order to destroy the protoss. It proclaimed "as I was your beginning - so shall I be...your end." The protoss came to realize that killing Kerrigan had allowed this to occur. The protoss were eventually exterminated at the hands of the zerg, and the zerg were also later eradicated. Zeratul wondered whether the "Fallen One" was a xel'naga.
Meanwhile, Jim Raynor and his friend Tychus Findlay had been hired by the Moebius Foundation, led by Dr. Emil Narud and Prince Valerian Mengsk, in order to acquire pieces of a xel'naga device which was much younger than most other such devices.
Raynor later made contact with Emil Narud, the head of the Template:Moebius Foundation, who revealed to him the artifact's xel'naga origin, explaining this was why Infested Kerrigan sought it as well. After collecting another piece, Raynor confronted Prince Valerian Mengsk, the heir apparent of the Terran Dominion. Valerian was not hostile, however. He told Raynor he would use the artifact to restore Kerrigan's humanity. Raynor agreed to work with him.
Raynor and the Dominion, leading an invasion of Char, used the xel'naga device to revert Sarah Kerrigan's infestation as well as destroy massive numbers of zerg.
"The greater whole" of the xel'naga was annihilated by the zerg. Only ruins and artifacts remained, some functional, of the ancient race in and around the Koprulu Sector.
The xel'naga left numerous artifacts on Aiur and other worlds, such as Ehlna, Roxara and Aridas.
Some artifacts can be considered dangerous, since they can manipulate the minds of terrans. At least some are alive in a sense (once being described as an "energy fossil").
These artifacts are sought by protoss. The Dark Templar especially have made efforts to track these down.
The Terran Dominion made attempts to buy some from rival powers. Even the zerg have used spies in an effort to locate artifacts.
The xel'naga left khaydarin crystals on Aiur, including the Uraj and Khalis.
The xel'naga left behind many temples. On Aiur, they built a temple to mark the spot where they first set foot upon that world.
The xel'naga left a temple on Shakuras, sitting on a nexus of cosmic energy,where their marks (such as darkening the planet and a temple capable of mass destruction) are more obvious.
Still more temples have appeared on worlds such as Nemaka, Aridas (where the powerful Argus Stone was stored)and Bhekar Ro; the latter disgorged a strange energy creature which collected protoss and zerg genetics for unknown purposes.
Valerian Mengsk, the Heir Apparent of the Terran Dominion, has expressed interest in these temples, and Dominion forces have engaged in combat in an attempt to secure such temples.
In addition, watch towers seemingly crafted by the xel'naga have appeared.
The xel'naga possessed the ability to create space platforms.
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".
At least some xel'naga buried and/or entombed their dead.
The xel'naga used a "universal natural" code the protoss termed "ara'dor", which means "perfect ratio". Terrans call it the Golden Mean, symbolized by φ ("phi").
In mathematics and the arts, two quantities are in the golden ratio if the ratio between the sum of those quantities and the larger one is the same as the ratio between the larger one and the smaller. The golden ratio is approximately 1.6180339887. The golden ratio is related to the Fibonacci sequence and the Golden Spiral.
Braken PropheciesEdit There are prophecies relating to the planet Braken, a protoss world with religious significance. These prophecies claim that "he who harnesses this world will someday wield a great power, sanctified by the ancient ones".
Kerrigan and Zeratul's RoleEdit Kerrigan bantered with Zeratul on Char as a prelude to combat, where Zeratul told her: "Your coming has been foretold... You are part of the culmination. But not the end of it. You shall show the way, the path that must be taken, the realigning of old truths no longer valid. Yours is not the hand, but your very existence provides necessary instruction." Kerrigan did not treat the pronouncement seriously at the time.
- Amon (a 'fallen' xel'naga)