Described as "the most powerful species the universe has ever known," the xel'naga have been referred to as "wanderers from afar," but it has also been described as being their true name. The protoss referred to the xel'naga as the "Ihan-rii", the "Great Teachers", the "Makers", and the "Guardians", during the Aeon of Strife, and prior to it, worshiped them as gods. Some protoss still refer to them as such.
According to protoss tradition, the xel'naga were peaceful, benevolent and utterly consumed with studying and propagating sentient evolution within the universe.
Originating from the Void, 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 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 would naturally come together and merge, forming the next generation of xel'naga and beginning the cycle anew.
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.
Using khaydarin crystals, the xel'naga carefully steered the delicate evolutionary processes of their budding species for millennia, keeping a detailed genetic history of each race. Their protracted experiments produced many intriguing deviants and mutations, but the races that they cultivated always fell short of their enormous expectations.
The following section contains timeline information which isn't necessarily in chronological order.
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.
Still driven to perfect their science of proto-genetic evolution, the xel'naga focused their frustrated efforts on Aiur, one of their engineered worlds. Pleased with the progress of its most advanced native life-forms, the xel'naga conceded that their latest creation was their first of all their experiments to evolve beyond the feral constraints of baser life-forms. They gave the new race the name of protoss, or 'the firstborn'.
The xel'naga slumbered within the Chamber of Ascension as they waited for the two destined races to assemble the Keystone and find their way to Ulnar. Meanwhile, Amon and his followers traveled to Aiur. Unsatisfied with the slow progress of the protoss, they accelerated their evolution and spent a millennium subtly guiding their steps. They even used devices such as the Kassia crystal to enhance their rate of reproduction. Eventually, Amon's xel'naga succeeded in leading the protoss to the state of total sentience and awareness. When the protoss became highly intellectual and introspective, Amon and his followers were exhilarated by their seeming success. Attempting to discern the full extent of protoss evolution, they made themselves known to the protoss and integrated into their culture.
Amon's xel'naga constructed a temple to mark the site where they first set foot upon Aiur. They also dug vast subterranean caverns.
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". Amon's xel'naga marveled at how driven the protoss were to plumb the mysteries of the universe around them. However, they reeled in frustration when the protoss tribes grew further and further apart. At first, Amon's 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.
Amon's xel'naga speculated that perhaps they had pushed protoss evolution too far. As individual egos arose to overpower the once primary communal link, many of Amon's followers believed that the protoss had lost their greatest strengths. With the loss of the primal psychic link, which the xel'naga considered the most fundamental element of their greatness, they deemed that the purity of form they sought to create had been sullied by a conflict of essence and thus decreed that the protoss were a failed creation as a result of their sentience being pushed too quickly. Amon's xel'naga were already done with the protoss, and needed to work on a new species. Hundreds of Amon's were killed in a protoss attack when they tried to leave, but they were fended off long enough for the greater number of their worldships to be launched. They left behind khaydarin crystals and at least one ship.
Amon and his followers traveled to Zerus, where they dismissed their tenets of physical form and focused chiefly on the pursuit of a distinct purity of essence. They labored to advance the evolution of the zerg, which was the most insignificant life form on Zerus until their proto-genetic manipulations enabled the zerg to survive the torrential firestorms and thrive.
The zerg ultimately developed the ability to assimilate other races into themselves, and Amon knew that they had achieved their goal when the zerg became conditioned to survive the harshness of space. However, the xel'naga were alarmed to discover that the original races assimilated by the zerg were almost unrecognizable after only a few generations of their inception. Amon desired their ability to steal essence, but the zerg were independent. Remembering their failure with the protoss, Amon's xel'naga attempted to waylay the potential hazards of differing egos by structuring the collective sentience of the zerg into a unified, amalgamated hive mind, creating a separate entity called the Overmind to control the swarm. They kept a constant watch on the Overmind via a psychic link.
Feeling an absence within the Void itself as the protoss created the Khala, the other xel'naga were disturbed from their slumber. Discovering Amon and his followers at Zerus, a war erupted among the xel'naga; in response, Amon unleashed the zerg. The xel'naga attempted to stem the tide of the ever-advancing onslaught, but the reinforced hulls of their worldships were battered by wave after wave of zerg swarms with no signs of abatement. The xel'naga defenses were overrun within a few hours and the greater whole of the xel'naga race was consumed, an action which threw the xel'naga plan into turmoil. A fraction of the xel'naga remained within Ulnar, however, and Amon himself was felled in the final confrontation. He was thrust back into the Void and twisted it with his hatred, though the zerg continued to feel his influence. One of his surviving servants thereafter sought a body to sustain Amon.
The Overmind assimilated some of the xel'naga's knowledge as it processed thousands of xel'naga into itself, and decided that the protoss would become the zerg's ultimate target.
When the Nerazim found a temple which the xel'naga had constructed above a nexus of powerful cosmic energies, they decided to remain on Shakuras to study it. During the Brood War, the temple grounds were infested by two cerebrates. The protoss scoured the infestation then secured the Uraj. On Char, several cerebrates locked onto the Khalis' energies and nested near it, but it too was retrieved by the protoss. The protoss used the crystals to channel the temple's energies. The resulting explosion obliterated the zerg on Shakuras.
Duran used genetic manipulation to create protoss/zerg hybrids on a thousand different worlds. At the end of the Brood War, Dark PrelateZeratul encountered Duran on a dark moon. He continued to seek answers for what manner of being Duran truly was, and what dark future he was working toward. 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".
After uncovering a revelation somehow related to the xel'naga, Zeratul sought additional clues to unravel the mystery. Both he and Sarah Kerrigan were drawn to Ulaan to uncover a prophecy, rumored to foretell the return of the xel'naga, that was divided into three fragments, each kept at separate shrines. Zeratul gathered the fragments and had them deciphered by the preservers of Zhakul.
Fearing the Keystone's power, Kerrigan dispersed the zerg to search for the various pieces. After Raynor's Raiders had collected the other Keystone fragments, it was assembled and the engineer corps moved it to the base of the primary hive cluster on Char. The terrans protected the artifact while it charged. It then extracted the essence that forged Kerrigan into the Queen of Blades. Narud used that energy to release Amon from the Void.
The xel'naga wished Kerrigan restored as leader of the Swarm, and had Zeratul (by this point serving their interests, or at least what he perceived as the greater good) guide her to Zerus, where she was reborn as the Primal Queen of Blades, free of Amon's influence. Xel'naga relics which housed genetic information were also collected by Kerrigan on Zerus.
Narud battled Kerrigan at Amon's temple, which he had built Skygeirr Station on top of. Alexei Stukov deactivated the five xel'naga temples that were amplifying Narud's power. Kerrigan killed Narud in the heart of the temple, but by this point Amon was alive again.
Zeratul, still searching for the last part of the xel'naga prophecy, came to a facility of Moebius Foundation for information and discovered that Amon had been revived on Atrias.
Zeratul, together with the forces of PraetorTalis, reached Atrias and faced the Tal'darim to access the Temple of Erris. Inside the temple, Zeratul discovered that Amon was communicating with HighlordMa'lash through a void catalyst. Zeratul destroyed the catalyst, at which point Ouros told him that the Keystone would "usher you unto hope," showing its location on Korhal. Zeratul escaped the temple, despite Amon's efforts to stop him.
The Keystone was taken by Moebius Corps. However, it was reclaimed by the Dominion at Bennet Port and handed over to Artanis.
The Keystone guided Artanis into the Alterian Rift to Ulnar, where he and Kerrigan entered the Temple of Unification and, in accordance with the prophecies, worked together to seek out the Halls of Revelation. Upon entering the Chamber of Ascension, however, they found that those who slumbered within had been slain.
Amon opened a portal to the Void, but the protoss were able to destroy the Void crystals which powered the gate, sealing the portal.
Returning to Aiur, the protoss destroyed Amon's host body, then used the Keystone to extract him from the Khala. Once their nerve cords were severed, Amon was thrust back into the Void.
Amon's planned to return as his hatred festered within the Void. Ouros called to Kerrigan, who led an expedition into the Void with Artanis and Commander Jim Raynor. Narud was defeated by their forces, then finished off by Stukov.
Ouros urged Kerrigan to merge with his essence and she agreed.
Amon sent his forces against Kerrigan, but she was defended by Raynor, Artanis and BroodmotherZagara while Ouros bestowed his essence.
With Ouros having given his life, the Infinite Cycle came to its end. Kerrigan ascended as xel'naga.
With help from her allies, Kerrigan destroyed Amon's void crystals and defeated Amon.
Xel'naga corpses were gathered by the Daelaam for study, who gained insight into their biology. The Terran Dominion was unable recover any bodies, but EmperorValerian Mengsk desired to request the findings of the protoss. The Zerg Swarm also gathered what xel'naga essence it could from the corpses at Ulnar.
As part of her parting gift to the Zerg Swarm, Sarah Kerrigan gifted OverqueenZagara with the ability for the zerg to choose their own destiny, as well as the location of a sentience species who could house xel'naga essence. Zagara, seeking to have the zerg undo the damage of the past decade, had the Swarm take only a sufficient number of this species, and tasked zerg evolution master Abathur create a new species named the adostra, which could regenerate life on planets, from mixing xel'naga and zerg essence. The life spawned on planets would also show signs of xel'naga essence within them. However, Abathur was displeased with the mixing of zerg and xel'naga as well as the peace Zagara desired, and also used the essence to create chitha, psionic creatures that could control nearby zerg. Abathur sought to have the Dominion and Daelaam destroy the adostra, sparking a war between the factions to return the swarm to its former state. However, his plot was revealed, and his chitha were destroyed.
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. 
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.
"Imagine the terrans obtained one of our fabrication matrices. Could their scientists ever understand their engineering? The task set before me is of a similar scale. The xel'naga were beings beyond our understanding."
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 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.
The xel'naga were not a playable race inStarCraft II, as only the original three races can be played. Storywise, they form the backbone of the trilogy.
The xel'naga may have played a different role in the trilogy originally. An early cinematic depicted Zeratul warning Jim Raynor of the xel'naga's linkage with alien artifacts, that they were the key to their return. For the release of Wings of Liberty, this content was replaced with Zeratul giving Raynor information about Kerrigan.
A development version of the map "Echoes of the Future" identifies the xel'naga as the ones who manipulated the Overmind to destroy the protoss: Overmind: The Xel'Naga had created me to eradicate their failed children - the protoss - in a battle neither species would survive. The Overmind also created Kerrigan to stop the return of the Xel'naga: Overmind: Yet my...daughter...remained. She alone would break the cycle. She alone would forge a future devoid of the hated Xel'Naga.
↑Overmind: "Behold, there is a temple, not far from here, that lies upon ground most hallowed. Though I have born witness to the passing of countless millennia, the temple which you must assault is older by far. For it was constructed by my creators, the Xel'Naga, and it marks the site where the Xel'Naga first set foot upon Aiur." StarCraft. Vivendi Games. Level/area: Full Circle (in English). 1998.
↑Overmind: "Now shall the events set into motion so long ago be made complete. For the Protoss, too, were created by the Xel'Naga. They were the first creation, gifted with a purity of form. And we were the second creation, blessed with a purity of essence. Indeed, our two species are but opposite facets of a greater whole. Soon shall our two races be made as one." StarCraft. Vivendi Games. Level/area: Full Circle (in English). 1998.
↑Zeratul: "The Zerg were indeed created by the ancient Xel'Naga, the same beings that empowered us in our infancy. But the Overmind grew beyond their constraints, and has at last come to finish the experiments they began so long ago." StarCraft. Vivendi Games. Level/area: Shadow Hunters (in English). 1998.
↑The Xel'Naga Watch Towers do not grant 'detection' of cloaked units, nor does it move around like an Observer. Thus, if that pack of Dark Templars come storming into your base and you have all your probes gathered around that watch tower, it's not going to do you any good... Karune. 2008-09-16. Karune, i have a question about observers. Battle.net StarCraft II General Discussion Forum. Accessed 2008-09-16.
↑The Xel'Naga Watch Towers have larger sight range than Observers, but do not offer detection. I definitely wouldn't say they are inferior, as they have crucial uses throughout the game, especially in the earlier tiers. With only a SCV/probe/drone, a player can activate a watch tower, letting you know when the enemy is coming to attack you and with that. This alone, could easily change the outcome of any early skirmish battle that happens. Karune. 2008-09-16. Karune, i have a question about observers. Battle.net StarCraft II General Discussion Forum. Accessed 2008-09-16.
↑Q: How much different are the Protoss from the Xel'Naga?
A: They're different because they don't have any art. We haven't really gone too far into defining Xel'Naga other than defining a few temples and things. We on purpose don't really want to showcase what exactly a Xel'Naga is, because who knows what the future brings and we may decide we want to do something more with them. The Xel'Naga, artwise, they don't have lots of art. Now, Dark Templar and Protoss, their schemes are sort of the same i.e. curving and arcing armor, but dark templar are a little more archaic. Their metals are platinum, silver and blue hue where the Protoss are more warmful, bronze even some ivories and things like that.Samwise Didier, Brian Sousa, Alan Dilling, StarCraft Legacy staff. 2009-08-23. BlizzCon 2009 StarCraft II Art Panel. StarCraft Legacy. Accessed 2009-11-19.