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What's already happened
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My interpretation of the Xel'Naga's 'purity of form and purity of essence' description is that like the Zerg, their minds exist as one, in a disembodied consciousness, but like the Protoss they are all sentient and capable of sharing their thoughts. They all agree with each other on all fronts, but are capable of multifaceted analysis and interpretation of the world, rather than argue in individuality like the Protoss, or serve the will of one like the Zerg.
In countless ages, they lived and died, genetically engineering their successive race and transferring their collective consciousness to them as their legacy. Before the Protoss, they merely slowly engineered one race, then became them. The current problems in the Galaxy stem solely from the problems they had with the Protoss - Instead of accepting their fate, the Protoss developed individuality and rebelled. Knowing they needed another way to to transfer their consciousness into the the vessels of the Protoss to continue their lives, they created the Zerg, who bent to the will of the Overmind.
At this point the Overmind had freewill, as was described in SC2 - Once the Xel'Naga believed they had perfected the purity of form for the Zerg, they created the first Xel'Naga of the next generation - The Voice. Though because it was created artificially, with an essence of free will it was scorned by the Xel'Naga. (Think the rift between the High and Dark Templar, but on a much more extreme level.) Because they knew now that to succeed them, their creations would have to come together naturally rather than by their designs, they impressed the Voice at KL-2, and removed the free will of the Overmind - giving it only the directive to assimilate the Protoss.
At this point, as we already know, they discover the Overmind has severed their psionic links, attacks them. They depart for beyond the stars, to await their new vessels. At this point, they cease observations of all the races in the Koprulu sector, knowing that it will happen eventually and naturally. They can wait.
Unknown to them, however, is that the psionic link was not severed by the Zerg, but rather by the Voice. He reached out through the darkness and into the Overmind. Using his unique power to resist the will of the Xel'Naga's consciousness, he severed the link and twisted the Overmind's primary directive to destroying the Protoss, rather than assimilating them. (This is why the Overmind's memory is tortured in SC2.) This is the beginning of the Voice's thousand year plan to exterminate the Xel'Naga for harming the natural order, and imprisoning it within a dieing star.
Reaching through the stars, the Voice saw the Protoss and knew that in their state of Strife they would be unable to resist the onslaught of the Zerg. So he took his second immortal mental slave - the Templar Khas, he who brought order to the Protoss tribes. With this order they would be able to resist the onslaught of the Zerg long enough for the Xel'Naga to return and be destroyed as collateral damage in their war.
Many years later, a twist of fate as Voice feels some of the Protoss forcibly leaving the Khala he created through Khas to always have a mental conduit to their entire race. In time, as Khas faded from memory, he reentered the Protoss limelight as the Templar Adun. The Voice uses Khas (Now Adun) to remove the Dark Templar to Shakuras, a world considerably more bathed in the powers of the Void, while keeping them from exerting their influence on resistance on the rest of the Protoss.
Always opting to make the best of the situation, the Voice uses the blanket of the void over Shakuras to whisper to the Dark Templar and empower them. Though he cannot control them directly as he can with Khalai, he can ensure them as unwitting allies in the war he has been orchestrating for centuries.
LATER, ON EARTH.
The Voice, reaching through the stars to discover more allies for his coming vengeance, saw the Humans. As an emerging psionic power, the Voice could communicate with some of them. He took his third immortal slave here - Human Doran Routhe - as his avatar on the Earth, whom he used to push humans in the direction of interstellar travel. Through Doran, the Voice ensured that rather than the ostensible destination of Ganthris VI, the sleepers would awake in the Koprulu sector where they would fit between the Protoss and Zerg.
In Terran history, Doran Routhe died in obscurity. In reality, he was continuously changed and adapted to new ends by the Voice. Eventually, that end would be Samir Duran. But more on this later.
The Voice, feeling the Overmind attempting to resist its directive, brings its Protoss avatar forth once more, as Tassadar, a short while later, unknown to the Voice, Kerrigan is taken as a means for the Overmind to free the Zerg from their slavery.
(Because all the Voice knows of the Zerg is through the sentience of the Overmind, he is blind to the free-willed and sentient Kerrigan. The Overmind takes her knowing that the Voice is planning to kill him through Tassadar, and that he won't be able to see her. He takes her in following the directive of the Voice to assimilate all races they become inimical with in order to strengthen themselves.)
Yada yada, story of StarCraft I, Tassadar, under the directive of the Voice, kills the Overmind under the Voice's assumption that he'd take direct control of the Zerg when it died. When he doesn't, he finds Kerrigan, and attempts to have the Cerebrates he controls reform an Overmind under his direct control in order to subvert her - But he is too late, he's lost control of them completely when the Dark Templar, free of his influence slay them.
Knowing he will need some sort of army directly under his control for the onslaught of the Xel'Naga that is inevitable, he takes his human slave, Duran (Who had be used as a source of information on Kerrigan, who he cannot directly commune with), and begins to forge more unnatural Hybrids. Though like him in many ways, they lack the sophistication of Xel'Naga engineering, and cannot be a vessel as the Voice would have been. They merely live to serve his whim.
After the Brood War, The Voice knows the way to control the Dark Templar, who have thus far resisted his whispers through the void is through manipulation of Zeratul, so Tassadar speaks to him from the husk of the Overmind as a trusted friend and ally, manipulating on behalf of his master.
Dr. Narud, of the Moebius foundation is the third incarnation of the Voice's slave Duran. He seeks the Xel'Naga artifacts with the power to remove Kerrigan as Queen on behalf of the Voice as he believes this will again allow him to regain control of the Zerg swarms.
Raynor uninfests Kerrigan, wrongly interpreting Zeratul's warning from the Xel'Naga, and falling to the manipulation of Tychus through Mengsk and Narud.
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What's going to happen - Heart of the Swarm
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As more of the Prophecy comes to light for out fearless band of heroes and ... Kerrigan, it becomes obvious to them now that the Zerg, despite the lack of a leader still move with purpose and order. The Xel'Naga come, awaiting their newest host, only to be unexpectedly launched in attack by the Zerg and Khalai, who are controlled by the Voice.
Kerrigan willfully reinfests herself, at the behest of Raynor, attempting to retake the Zerg from the Voice, as she knows the Xel'Naga are the only ones who can destroy him.
Meanwhile, the Terran Dominion, being retarded, launches an assault of these new alien invaders. Raynor's Terrans and The Dark Templar struggle to defend the Xel'Naga on the other front from the Khalai and Hybrids.
The end leaves the Xel'Naga in war with the Terrans, the Voice's Protoss and Hybrids fighting the Raiders and Nerazim, and Kerrigan fully consolidating her power.
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What's going to happen - Legacy of the Void
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Zeratul, seeing the Protoss race in shambles urges the Khalai to severe their psionic gestalt as the the Dark Templar did in order to free themselves from the influence of the Voice.
The Xel'Naga, at the behest of Kerrigan, begin to assault the Hybrids and Voice across many worlds.
Raynor, Zeratul, and Valerian fight to fend off the Dominion from attacking the Xel'Naga, as they view them as a threat to Terran life. Cliffhanger ending as the Voice is defeated and the Xel'Naga again return to their ancient task - Giving Kerrigan's swarm the living directive of assimilating the Protoss that it once had.
None.
