It began with the vast, galaxy- consuming war between two machine civilizations known as the Abominor and the Silentium. Their conflict sprang from something organic species find hard to understand, ridiculous, really: the matter of symmetry. The Silentium believed it to be sacred; whatever shape their cybernetic bodies were constructed in, whether it was a sphere, cube, or any other geometrical shape, was built in perfect symmetry. The race that had created them had valued this when they had brought the very first of their kind into being. They'd been extinct for millennia, but if there was anything the Silentium had kept of their makers, it had been that concept.

The Abominor, on the other hand, were the polar opposite; they'd been brought into being by a humanoid race as intelligent mining machines, built for the sole purpose of gathering resources from anywhere in the galaxy as efficiently as possible. For this reason, they'd been programmed with the ability to graft new attachments, extensions, and appendages onto themselves however they saw fit. Like the mysterious creators of the Silentium, the Abominor's original manufacturers had disappeared from the galaxy since before they'd ever encounted another sentient race. Some believe that they had been made by an ancient race known as the Xel'naga, but that was pure speculation.

Since the disappearance of their creators, the two machine civilizations had gone in two very different directions: the Silentium used their resources simply to explore, map, and catalogue the various life-forms of their galaxy. The Abominor, on the other hand, continued with their original programming to the point that they became a threat to the galaxy's organic life. They stripped entire worlds of their mineral resources with no regard to any beings who might have been previously living there, often rendering planets that had been previously full of life uninhabitable, grafting the metals they found to their own bodies to the point where some of them grew to the size of small moons. They had little regard to symmetry or really any system of how they did this. The result: massive, monstrous-looking, starship-sized constructs that looked like the result of a collision of metal-rich asteroids.

The Silentium had no real regard for the organic life of the galaxy; they were, however, in their cybernetic way of thinking, offended by what the Abominor were doing to the galaxy at large and by their very appearance. So when the Abominor, oblivious to or in spite of the power of the Silentium, attempted to assimilate the other machine civilization's homeworld, the Silentium attacked, and thus began a galaxy-wide war between the machines. This war was fought by soldiers who knew no emotion, neither hate nor fear, simply their perception of one another as threats. Entire worlds were destroyed in the crossfire. The machines considered all organic life the way most humanoid creatures in the universe see the tiny insects of the worlds they inhabit, that is, as being completely insignificant. Because of this, they saw no reason to try to keep their battles in deep space. Silentium attackers would, in fact, attack a planet in the midst of being "harvested" by the Abominor, or would obliterate entire star systems to prevent their enemies from taking resources from them. This is what almost happened to Yuuzhan'tar, homeworld of the Yuuzhan Vong.

Yuuzhan'tar was something neither the Silentium nor the Abominor were prepared to encounter: a truly sentient planet. While it is not unheard of, and certainly not impossible in a universe of endless possibilities, a sentient planet is obviously a very rare and unlikely thing anywhere. The Silentium and Abominor certainly had never encountered such a thing anywhere they'd been before, and understandably failed to recognize Yuuzhan'tar for what it was until it was too late.

When the Abominor attacked, Yuuzhan'tar was ready. It had taught its inhabitants, the Yuuzhan Vong, the secrets of interstellar space travel centuries before and had encouraged them to "leave the nest," so to speak, and explore the universe around them in organic spaceships that had been grown from the homeworld into independent organisms capable of sustaining comfortable living conditions inside themselves for their crew. This also, unintentionally, provided Yuuzhan'tar with an early warning system when several of its outlying colonies were destroyed in the crossfire of massive battles between the machines. When the living world realized the true extent of the threat to itself and its people, it recalled all the surviving colonists and, for the first time in its existence, began to grow ships built for war.

This was against anything Yuuzhan'tar had ever done in its history. Before this, the living world and its people had valued all life and had never commited any acts of intergalactic aggression. Not that they'd ever had the opportunity- this galaxy was an old one, as galaxies go, and most of the species that had once inhabited it had run their course and disappeared in the course of several devastating wars that had been fought on a galactic scale in ancient times. In fact, because of the way they had been hidden and sheltered from the terrors of the galaxy by their living world in that chaotic age, the Yuuzhan Vong were the very last race in this galaxy to achieve spaceflight.

In the situation it now found itself in, Yuuzhan'tar and its people had no choice. They fought back against the machines with the unique form of biotechnology Yuuzhan'tar itself had developed, and because it was so unlike anything either the Silentium or the Abominor had before encountered, they were victorious.

Yuuzhan'tar itself was content to leave things as they were- the machines had been shown what Yuuzhan'tar was and would not trouble it or its people again. The Yuuzhan Vong felt differently. They had been changed during their time out in the galaxy. They had become more independent, more adventurous… and more vengeful. They saw the machines as abominations, a mockery of everything they were. So they took the war to them, retaking their territory and, over the next few centuries, the entire galaxy from the machines. They made no distinction between Abominor or Silentium- both had tried to destroy them and their homeworld. Ultimately, they were successful in destroying almost every machine in the galaxy. The only ones to escape were the ones who found a way to escape the galaxy itself, embarking on a millennia-spanning journey to another one.

The Yuuzhan Vong had paid a price for their crusade, however. Their sentient homeworld, Yuuzhan'tar, had literally evicted their entire species, forcing them out into the galaxy forever. This forced the Vong to adapt to living their entire lives on vast worldships, organisms similar to the coralskipper ships Yuuzhan'tar had created for them in the beginning, just bigger. Quite a bit bigger, actually- on average, they equaled the size of a small moon, like the monstrous Abominor. This was because, in a galaxy scarred by the machine wars as well as the great wars in which the spacefaring races that had existed before had destroyed themselves, nearly every inhabitable planet had also been destroyed at one point or another.

For a time, the Yuuzhan Vong were the absolute rulers of the galaxy. They discovered terraforming, which allowed them to convert worlds that had been rendered uninhabitable into near clones of Yuuzhan'tar- as far as the plants and animals went, at least. They were never able to understand the nature of the world they'd come from, never able to recreate it in any other world. Their civilization went into decay, and their unified leadership fractured. Warlords set up military bases in different star systems and began to fight each other for control. It was during this time of discord and chaos that the Xel'naga returned and brought about the end of the Yuuzhan Vong empire and of all life that remained in that galaxy.

The Xel'naga had been the creators of the Abominor, as well as the planters of the seed that became the sentient mind of Yuuzhan'tar itself- which, incidentally, was the world they had originally come from as well. For hundreds of thousands of years, in this galaxy as well as others, the Xel'naga had, as a species, sought to create the ultimate life-form, a being without weakness, without flaws. Their obsession had led them to play with the fates of entire races, of civilizations, and had ultimately led them to another galaxy where they had, or so they believed, come closer to their goal than ever before. They now returned to their galaxy and found it ruled entirely by one race; one similar to themselves, related, even, since they were from the same planet, no matter how different it was now than it had been before they left, so many millennia ago, but different. The Xel'naga saw the Yuuzhan Vong as being inferior, and demanded their fealty. The Vong, on the other hand, saw the vast starships the Xel'naga had undertaken their journey between galaxies in, and proclaimed them infidel servants of the machines.

The Xel'naga saw the war that was inevitably about to occur as an opportunity. They had spent their millennia in that other galaxy in pursuit of the same goal as ever: the ultimate life-form. They now returned to their own galaxy after developing and combining the most deadly parasites and mightiest predators that galaxy had had to offer. They had allowed a terrible parasite capable of consuming entire worlds known to the inhabitants of that galaxy as the Flood to combine with a previous creation of theirs: a race of creatures who were both predator and parasite, conquering army and contagion, that they had called Zerg. They had also, after that terrible combination had been made, thrown another ingredient in the mix; a terror they had not brought into being, but that rivaled the worst of the Zerg/Flood swarm's combat forms: a creature known to the inhabitants of that galaxy by only one name- xenomorph. These naturally-occuring creatures had been that galaxy's interstellar equivalent of sharks.

The result of this lethal mix had been a swarm of creatures so deadly the Xel'naga had been forced to destroy them all, keeping only some genetic samples which they'd programmed with a biological failsafe: all they had to do, when they had no more use for the creatures, was administer an airborne virus designed specifically to target the creatures. So, instead of a long, drawn-out war to retake the galaxy from their genetic cousins, the Xel'naga unleashed this biological horror on the Yuuzhan Vong.

The Yuuzhan Vong had no defense against such creatures. The swarm was capable of assimilating any biological weapon the Vong could throw at them. Using spores they generated for this purpose, the creatures could infect and turn living ships and the ecosystems of entire worlds against the Vong before they even arrived, so that the conquest was nearly complete by the time they did. Because the entire Yuuzhan Vong civilization relied entirely on biotechnology, every weapon they had could be assimilated and turned against them. The only thing some especially smart military leaders could do was to leave the galaxy entirely before the swarm reached them. By the end of it, these were the only survivors of the Yuuzhan Vong race.

This also proved to be a double edged sword that ended up being turned against the Xel'naga. They were able to neutralize a good amount of the swarm using the genetic failsafe they'd programmed into them, but because they hadn't taken the time to really analyze the nature of the Yuuzhan Vong's biotechnology, they failed to realize that they had once again given their creations the means for space travel- a mistake which proved, in the end, to be their undoing. As the Xel'naga seeded the sterilized worlds and all the assimilated Yuuzhan Vong ships they could find with the biological agent that neutralized the swarm, the surviving creatures fled further into former Vong territory, ultimately discovering Yuuzhan'tar.

There, on the living world that had given birth to the races that had brought about the end of all life in the galaxy, the swarm found something unique: a consciousness that, after being assimilated into the swarm, was able to transform them into a terror never before seen in the universe. After absorbing the life and consciousness of Yuuzhan'tar, the living world, the swarm now had a sentient consciousness behind it… a guiding force. The essence of Yuuzhan'tar was gone, save for one small seed of itself it had somehow flung out of the galaxy before it had met its doom. And in absorbing Yuuzhan'tar, the swarm had been rendered immune to the one thing the Xel'naga had programmed a weakness to into them.

Now, the creators became prey. The most ancient civilization of that galaxy was the last, and its people died the most terrible deaths, ripped apart by terrors born of their own science. Their very memories went to feed the new entity that had been born of the living essence of Yuuzhan'tar at the heart of the swarm, and entity the swarm knew as the Hive Mind.

After the assimilation of the Xel'naga, the swarm they had created and which had been brought into its ultimate glory by the formation of the Hive Mind, became the sole inhabitants of the empty galaxy. If there was one weakness of the swarm, however, it was that it/they were by nature consumers only- they could not produce resources to replace what they had consumed. So, after stripping every last world and assimilating every last bit of biomass that galaxy had to offer, the Hive Mind turned its perceptions outward. It looked with hunger at the nearby galaxies, crawling with life, ripe for the harvest. And so did the tyranid swarm leave that empty, lifeless galaxy behind forever. The Shadow in the Warp was born, and the Devourer of Worlds prepared for the invasion of a galaxy ripe for the picking: ours.