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Twisted incarnation of the xel'naga
Powerful psionic emanations when active
The xel'naga regularly conducted experiments on other species as part of their life cycle. Two species, one with the "purity of form" and another with the "purity of essence", would merge naturally to create a new iteration of xel'naga. This process had occurred numerous times.
To this purpose, the last incarnation of the xel'naga uplifted the protoss and zerg, intending for them to lead to the culmination of another iteration of the cycle.
However, others realized the protoss and zerg could be merged in such a way to produce another type of being: the hybrid. Contrary to the xel'naga's intent, the hybrids were the result of a perversion of the process. The Dark Voice actively sought to create hybrids to facilitate its plan to remake creation in its own image. Overt knowledge of the hybrids began spreading among the terrans, protoss, and zerg, after the Brood War, and most who knew understood the dire threat the hybrids posed to the status quo.
Hybrids possess a Consume DNA ability which inflicts harm on whatever it is used against. Hybrids would preferentially attack protoss and zerg with it and in an apocalyptic future, they used this ability to finish off the Zerg Swarm.
The Grand Experiment
The xel'naga were a long-lived species but eventually their time would come to an end. To stave off extinction they devised a way to increase their numbers by uplifting and peacefully merging over eons two species. If one species was "pure of form" and the other "pure of essence" then they believed the merged result would be a new incarnation of the xel'naga.
The xel'naga first uplifted the protoss of Aiur to be pure in form, before departing to the world of Zerus and beginning to uplift the original zerg of Zerus, which they considered to be pure of essence. as part of their plan, the xel'naga created the Overmind, a collection of zerg sentience, in order to enforce a purity of essence.
The Overmind's free will was undermined by an overriding directive to destroy the protoss. This was part of a greater plan by a being known as the Dark Voice to remold creation in its own image. The Overmind defied the Dark Voices manipulations, but could do little to resist. It would later attempt to undermine the Dark Voice by creating the Infested Kerrigan as a way to resists its directive.
In the meantime, the Overmind ordered the Swarm to attack the xel'naga worldships over Zerus. This threw the xel'naga plan into turmoil. The Swarm then embarked into space to "complete" the xel'naga's work by absorbing the protoss. The protoss became aware that it wished to finish the xel'naga experiments. Tassadar destroyed the Overmind, believing it would "run rampant throughout the stars, consuming all sentience – all life."
Bhekar Ro Experiment
The xel'naga constructed a temple on the world of Bhekar Ro, which came to be settled by terran colonists. Shortly after the Great War, the temple emerged. A battle between Terran Dominion Alpha Squadron, the protoss 7th Fleet and the zerg Kukulkan Brood erupted over the temple, which absorbed protoss, terran and zerg into itself, as well as the energy of a nuclear weapon.
An energy being emerged from the temple, and it continued to absorb protoss and zerg, combining their genetic material within it as part of its maturation process. However, it released the absorbed terrans unharmed before flying away.
Other such energy creatures have been observed, but they did not absorb any protoss or zerg.
Ulrezaj, the dark archon terrorist, created genetically enhanced zerg as part of his plan to drive the protoss of Aiur from Shakuras. He would use warped khaydarin crystals created from Dark Templar and zerg energies, as well as genetics stations, to modify zerg to become more powerful. These zerg would be under his control, and would be immune to the effects of the xel'naga temple of Shakuras. This aspect of Ulrezaj's plan did not come to fruition as a small elite force of protoss, including a Dark Archon Hero, destroyed the warped crystals.
Ulrezaj created strange creatures which he kept in vats in an underground city underneath Aiur. Their true nature is unknown, but they have psionic powers and his loyal Tal'darim were taught to trap zerg for him. The preserver Zamara suggested they may be a type of hybrid.
On the direction of his superiors, Samir Duran worked to produce a perfected protoss/zerg hybrid. By the end of the Brood War he had established development bases on multiple worlds and successfully created numerous specimens. The work was accelerated by Sarah Kerrigan's assimilation by the zerg. Duran claimed that "this creature [was] the completion of a cycle. Its role in the cosmic order was preordained when the stars were young."
One base on a dark moon was discovered accidentally by the Dark Templar Zeratul. He was en route from Char having killed the second Overmind for Kerrigan, and slain Raszagal to save the Matriarch from Kerrigan's influence. The prelate sought to find and rendezvous with Praetor Artanis and thought the protoss energy signatures emanating from the moon were from protoss stragglers or the praetor's forces.
Zeratul led a party to the surface and found not protoss but the hybrid development base instead. He was puzzled by terran mercenaries seemingly using protoss technology, and the protoss, zerg, and infested terran specimens in stasis cells. Zeratul was horrified when they discovered the hybrid specimen's cell. The prelate was disturbed further when he was contacted by Duran, who told him the experiments were being sponsored by a greater power and specimens were secreted on multiple worlds. Zeratul destroyed the hybrid before departing.
Zeratul was unable to explain his unnerving discovery and kept it secret but embarked on a solitary fact finding journey. In 2503, Zeratul encountered the preserver Zamara, and they exchanged information. He told her about the hybrid he saw, and she told him about the xel'naga cycle. According to Zamara, Samir Duran's experiments are a "perversion" of what was to be, and if his attempts to "unify" the species become successful, it will result in "a monstrous and powerful perversion of both protoss and zerg will be set loose upon the universe, and all that we know and cherish will fall in their wake".
At the Protoss Advance/Experimental Facility, protoss scientists conducted an experiment with creep, infecting it with a virus. The creep began to act strangely, attacking two of the zealot guards sent to oversee the dangerous project. While one guard killed himself, the other became infested despite his connection to the Khala. The creep used its Khala-influencing abilities to drive the protoss mad while the hybrid zealot went on a rampage, but it was frozen by the last surviving scientist, an act which destroyed the facility.
The Terran Dominion conducted hybrid experiments at a secret facility orbiting Castanar. The gene splicing technology was far more advanced than anything the terrans had, suggesting the Dominion had help.
Discovering the facility, Raynor's Raiders attacked it, discovering numerous hybrids being "cloned" within. They destroyed the fusion core, causing a containment field to weaken and a particularly powerful hybrid reaver emerged. The Raiders fled as it went on a rampage, attacking protoss, zerg and terrans alike.
The Second Great War
On the eve of the Second Great War, the Dark Voice began moving its hybrids and executing its grand design for control. The hybrid destroyer Maar went to the protoss archive world of Zhakul and attempted to leech the energies of preservers. It also subverted the planet's defenders, the Zhakul Guardians, into protecting it. The hybrid was ultimately stopped by Zeratul who came to the planet to consult with the preservers regarding the ancient xel'naga prophecy.
- Kerrigan to Zeratul(src)
The Overmind's apocalyptic vision of the future remained unknown at large until Zeratul sought out the creature's memories. The dark templar sought greater understanding to a dire prophecy from Ulaan foretelling the end of the xel'naga cycle. The prelate learned of the Overmind's vision through Tassadar's spirit. In it, the zerg were enslaved by the Dark Voice and the hybrids after the death of Kerrigan. The terrans were soon annihilated. The combined hybrid-zerg force destroyed the protoss, before turning on and eliminating the zerg as well.
- Castanar hybrid
- Maar (Zhakul ruler)
- Ruom (created through PAX experiment, described as an "infested protoss" and significantly different from other hybrids)
- Test subject twenty nine (dark moon)
- Test subject 405-GLTV3 (Castanar experiment)
- Blizzard went through thousands of hybrid designs over the course of StarCraft II's development. Out of the thousands of forms pitched, only a few were locked down.