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Twisted incarnation of the xel'naga
Powerful psionic emanations when active
The protoss/zerg hybrid is a perverse fusion of the protoss and zerg species.  Described as "false xel'naga" , they were the ultimate weapons of the fallen xel'naga Amon and were key to his plan to remake creation in his image.
The xel'naga regularly seeded the universe with life as part of their Infinite Cycle. Over many generations, two species, one with the "purity of form" and another with the "purity of essence", would come together naturally. Their xel'naga progenitors would then transfer their essence to them, giving rise to a new generation of xel'naga, which would repeat the cycle.  
The xel'naga Amon, however, viewed this cycle as corrupt and flawed. He desired to remake the universe in his own image.  To this end, he and his followers violated the xel'naga's rule of non-intervention, and uplifted the protoss and the zerg, with the intention of merging the two species to create the hybrid.  Contrary to the xel'naga's intent, the hybrids were the result of a perversion of their Infinite Cycle, as their creation would not lead to the rebirth of the true xel'naga. 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 universe.
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 thoughts of hybrids have been described as "red hate".
The Grand ExperimentEdit
The xel'naga were a species that reproduced by seeding the universe with life. In time, two species would emerge, one that was "pure of form" and the other "pure of essence". Inevitably, these two species would, through peaceful means, come together and merge enabling their xel'naga creators to pass their essence onto them, thus giving rise to the next generation of xel'naga. Although the xel'naga watched over the younger races, they never interfered with their evolution or development. 
One xel'naga, Amon, viewed this cycle as corrupt and saw it as the source of all suffering in the universe. After gathering a group of like-minded followers, Amon set out to find two races he could uplift and groom to create the perfect life form that would end the cycle. He first uplifted the protoss of Aiur to be pure in form, before departing to the world of Zerus where he uplifted the zerg, which he considered to be pure of essence. As part of his plan, he 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 Amon to remold creation in his own image. The Overmind defied Amon's manipulations, but could do little to resist. It would later attempt to undermine Amon by creating the Queen of Blades as a way to bypass its directive.
When the other xel'naga discovered Amon's experiments and tried to put a stop to them, Amon unleashed the Overmind and the Swarm against the xel'naga worldships over Zerus. The Swarm wiped out the majority of the xel'naga, throwing the Infinite Cycle 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." Although the zerg desired to assimilate the protoss, the Khala and the Void protected the protoss from infestation, preventing the zerg from absorbing the protoss into their fold.
Fulfilling the machinations of Amon, Samir Duran worked to produce a perfected protoss/zerg hybrid as part of a stepping stone in order to create a much more powerful host body in order to sustain his master. 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 a 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 and UlrezajEdit
Ulrezaj created strange creatures which he kept in vats in an underground city underneath Aiur, sometime after the Fall of Aiur. Their true nature is unknown, but they have psionic powers and his loyal Tal'darim were taught to trap zerg for him. The creatures were kept secret from the Tal'darim, whom Ulrezaj was draining for their energy.
Zeratul encountered Zamara shortly afterward, and they exchanged information. He told her about the hybrid he saw on the dark moon, 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".
The Second Great WarEdit
The hybrid were released on the eve of the Second Great War. 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 which Zeratul had recently discovered.
Terran Dominion ExperimentsEdit
By a few years prior to the Second Great War, the Terran Dominion had initiated a Hybrid Breeding Program, based out of Skygeirr. The Dominion also 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.
Transfer of EnergyEdit
Moebius Foundation ExperimentsEdit
Prior to 2504, Dr. Emil Narud of the Moebius Foundation created at least three hybrids, something which he did not keep secret from his benefactor, Valerian Mengsk. However, Narud concealed from him how far his experiments had gone. Each of the hybrids had a markedly different appearance. They were not a match for an uninfested recovering Sarah Kerrigan.
In a conflict following the Battle of Char, Narud betrayed Valerian Mengsk by unleashing the hybrids against him. Arcturus Mengsk also knew about these hybrids. Arcturus claimed that Narud could pacify the hybrids with a specific psionic signal.
Hybrid dominators were encountered by Kerrigan and her forces on Skygeirr Station, led by Narud. They broke free of containment, overcame the Dominion forces assigned to oversee them, and remotely attacked Kerrigan through psionic means. However, the zerg were able to overcome them.
Narud made a final stand at a series of xel'naga shrines further in the station. His loyal Tal'darim faced Stukov and Kerrigan, and many hybrid reavers fought alongside the Tal'darim. Stukov led the zerg to victory, however. Kerrigan faced Narud in single combat, where Narud revealed his shapeshifter abilities. Kerrigan stabbed Narud, but as the shapeshifter lay dying, he revealed that Amon had already been revived.
Legacy of the VoidEdit
Prelude to OblivionEdit
In the aftermath of the Second Great War, the hybrid began to awaken and seize control of the Moebius Corps and Moebius Foundation terrans that were creating them, turning them into thralls. One such facility was the Revanscar facility, where the majority of hybrids were created.
A number of hybrids controlled Moebius Corps troops at a base. Zeratul and Kerrigan discovered them. They killed many of the thralls and Zeratul's troops slew the hybrids. Zeratul was there to find Amon's rebirth place, and believed that slaying the hybrid was meaningless, as he needed to find the xel'naga to defeat Amon instead.
Zeratul and Praetor Talis's forces later attacked the Temple of Erris on Atrias. The Tal'darim had failed to keep Zeratul out, and attacked Talis's forces outside the temple. Hybrid joined in the assault. Within the temple, Zeratul fought more Tal'darim troops, including hybrid. The Tal'darim highlord, Ma'lash, seemed to hold hybrid reavers in high esteem, but Zeratul could still fight them. Amon emerged from the void, summoning shadows of the void to attack Zeratul along with hybrid. Zeratul was forced to flee before Amon destroyed the temple.
Meanwhile, the Daelaam launched their first attempt to reclaim Aiur. Hierarch Artanis was warned by Zeratul of Amon's plans, but chose to ignore him. During the battle to reclaim the warp conduits on the planet, Amon's hybrid forces began to coordinate the Amon Brood zerg attacks against the protoss. When the Daelaam sieged the final warp conduit, hybrid forces appeared to directly intercede in its defense. The appearance of the hybrid convinced Artanis to act against Amon, and prepared his forces for the coming war. However, Amon seized control of the Khala, and sent hybrid forces alongside corrupted protoss and Amon Brood zerg against Artanis's few uncontrolled protoss forces. He then released the hybrid alongside of his armies to begin his galactic conquest.
The following section contains timeline information which isn't necessarily in chronological order.
During Amon's assault on Shakuras, Amon held his hybrid forces back and used zerg forces to test the Daelaam defenses of the planet. During the final assault on the xel'naga temple of Shakuras, Amon sent his hybrid through the planet's warp gate and used them to besiege the temple. The assault was unsuccessful in destroying the temple in time, and Amon's forces on the planet were annihilated when the temple's overloaded phase prism destroyed the planet.
Hybrids were also present during Moebius Corps's siege of Korhal. When Moebius Corps retrieved the Keystone and retreated to Bennet Port, hybrid appeared to aid in its defense, and Hybrid dominators channeled void energy into the Keystone to stun all terran forces. This attack did not effect the Daelaam forces aiding the Terran Dominion, which were able to push through the energy surges and retrieve the Keystone. The hybrid and Moebius Corps were pushed from Korhal.
When Artanis found his way to Ulnar with the intention of resurrecting the slumbering xel'naga there, he found Sarah Kerrigan engaged with hybrid forces. The two fought them off, and she revealed she had been fighting them off for days. The two fought through hybrid that barred their path to uncover the secrets of the ruins, but found the xel'naga already dead when they arrived.  When Amon opened a rift to the Void to destroy Artanis and Kerrigan, he sent hybrid forces to stop them from destroying the void crystals powering the portal. The defense failed, and the crystals were destroyed. 
When the Daelaam attempted to reawaken the Purifiers on Cybros, hybrid forces were sent to reinforce the Amon Brood that landed on the platform. These were destroyed alongside the zerg when the Purifiers awakened and cleansed the station.
During the Daelaam's attempts to instate Alarak as Highlord, hybrid forces guarded the Guardians of Ma'lash and supplemented the Tal'darim forces. When Alarak finally fought Ma'lash in Rak'Shir, hybrid channeled their psionic energy to Ma'lash to help him defeat Alarak. However, this failed, and Alarak revealed that Amon's promise to the Tal'darim that they would ascend into hybrid was a lie. As such, he commanded the Tal'darim to destroy Amon and his forces.
During the conflict, Matriarch Vorazun sent her best dark templar to track down the facility where the hybrid were being created. Her scouts eventually discovered Revanscar, and the Daelaam set a course for the facility. However, Moebius Corps battlecruisers trapped them, and hybrid forces began to board the Spear of Adun. Phasesmith Karax lead the assault on the facility itself, destroying its power generators. The Spear of Adun fired on the facility, ending Moebius Corps as a threat and destroying their hybrid breeding operations.
As the Daelaam returned to reclaim Aiur, they first sought to delay the corrupted Golden Armada's arrival by destroying the planet's psionic matrix. After the first khaydarin crystal powering the matrix was destroyed, hybrid defenders were sent alongside corrupted Khalai protoss to defend the second. Karax and Talandar reclaimed a force of robotic units, and used them to break through the hybrid lines and destroy the second crystal. At the site of the dead Overmind, Amon had created a hybrid body from protoss flesh combined with that of the Overmind. Hybrid attempted to stop the Daelaam from destroying the void crystals defending Amon's host body, but the Daelaam broke through and unleashed a massive orbital bombardment on it, destroying Amon's host and leaving only the Khala left for him to inhabit. 
- Kerrigan to Zeratul(src)
The Overmind's apocalyptic vision of the future remained unknown at large until Prelate Zeratul sought out the creature's memories. He 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 Amon 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.
- Amon (temporary host form)
- Castanar Hybrid
- Maar (Zhakul ruler)
- 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. The cut designs were "aquatic and graceful," intended to convey an alien look that was more advanced than the protoss and zerg.
- The idea of hybrids infiltrating terran governments and precipitating civil wars has been raised. According to Chris Metzen, the idea may be pursued, but as of 2011, there are no plans to do so.
- Cut lines for Heart of the Swarm indicate that hybrids would have been encountered outside Skygeirr in a pre-release version of the game.