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 the 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 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".
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.
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.
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 a overriding directive to destroy the protoss. This was part of a greater plan by the Dark Voice to remould creation in its 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.
"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. 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. 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 have 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.
Uplift (specifically biological uplift) is the act of an advanced civilization helping the development of another species. The xel'naga carried out this process multiple times as part of their cycle of reproduction, choosing two species that possessed purity of essence and purity of form. After seeding life in the Milky Way, the xel'naga returned to the jungle world of Aiur and began uplifting the protoss. Over the eons, they watched and secretly molded them and then moving on to the zerg, but their plans went awry. The xel'naga utilized both genetic engineering and guidance to enhance species, in the case of the zerg it was heavily emphasized on the genetic aspects rather than the cultural and mental ones. There are two ways of uplifting with several branches each, the indirect one used in the creation of the protoss who were largely left to mold their own civilization while secretly being aided and enhanced by the xel'naga. The direct uplifting was used in the creation of the zerg, almost every step of their evolution was cataloged and monitored. The xel'naga molded them into creatures without free will and bound to a single Overmind.
Worldships were "mighty" xel'naga ships. Worldships were capable of landing on planets, as well as remaining above planets when the xel'naga didn't want the inhabitants to notice them. At least one worldship was attacked by the protoss who jumped on it as it took off; the protoss fell to their deaths. Xel'naga ships were protected by reinforced hulls and sharp crystals. This failed to protect the xel'naga fleet against the attack by the zerg. One worldship was abandoned in the Sigma Quadrant. During the Second Great War it was occupied by the Tal'darim and protected by rip-field generators. The rip-fields destroyed unshielded objects.