On the Rangdan, Those Dread Parasites and Devourers
By author Perfidious Albion
The Rangdan Cerebravores are a species of parasitic xenos who reproduce by preying upon other sapient species. After mating, a female Rangdan Cerebravore will attack a member of another species, violently subdue him or her, knock him or her unconscious, and inject her newly fertilised egg inside the brain. There, inside her victim's brain, her embryo grows to maturity, feeding on oxygenated blood flow, learning through neurons' electrical impulses and absorbing brain matter into itself. At first, the embryo is almost unnoticeable, though the right scan can reveal its presence. If seen early enough, a medical intervention can even remove it without completely certain death for the victim, if it is done within the first six hours. For days, the victim undergoes shaking fits, hallucinations, and increasingly disturbed and schizophrenic behaviour. At last, four days after the egg-laying, the brain has been fully incorporated into the new organism.
The infant Rangdan cracks the skull like an egg, bursts out of its bony prison, and skitters away into the outside world. All that is left behind is the corpse of a victim, with a disturbingly empty skull, drained of everything within.
The new-hatched Rangdan is a near-spherical spiderlike creature, smaller than a cat, yet it already knows everything its victim knew, as well as everything its mother knew, and thus its mother's mother and its mother's mother's mother. It is for this vile habit that the Rangdan are known as 'Cerebravores'.
A new-hatched Rangdan has no armour, nor even skin. It is essentially a stolen brain with six spiny legs for locomotion. The first priority for this creature, known colloquially among Imperial warriors as a 'Skullcracker', is to get away. Avoid humans, or whatever other species it has infiltrated. Find a nice dark hole to hide in and grow. In this extreme youth, Rangdan are easily slain by even primitive weapons such as a stubber's bullet… if they can be hit. The true protection of Skullcrackers is that they move so fast it is hard to hit them. And in the shock and horror of a Rangdan Skullcracker bloodily bursting out of the victim's skull, people are understandably disturbed. The Skullcracker is usually able to flee the room before enough of the room's occupants have gathered their wits and started shooting for them to have any decent chance of hitting it. Then it escapes into the dark and cavernous recesses of a starship. Hunting Skullcrackers in a starship is an Imperial Army spaceman's worst nightmare. They can hide anywhere, they may start small, but they grow quickly. And if there is more than one, a male and a female, they may breed.
In six more months, new-hatched Skullcrackers grow up into true Cerebravores, powerful beasts of war. An adult Rangdan Cerebravore is a black-armoured behemoth of more than human height, towering two and a half metres tall from toe to shoulder, and covered from head to toe with metallic black scales. It has no head on its shoulders, but a grotesquely thick torso, for the stolen brain is buried deep within its chest, as far as possible from the surface of its body and thus the outside world, for better protection. Rangdan Cerebravores do not flee and skitter like infant Skullcrackers. They walk upright on two legs, both immensely muscular. The other four of the Skullcracker's infant legs have turned into mighty arms, all tipped with razor-sharp claws. Each arm also has a mouth on its hand.
How did these terrors come about into the modern galaxy?
The Rangdan evolved from parasitic brainworms on the planet Rangda, or R'angd as the Rangdan call it in their own tongue. Old Rangda lies in the galactic centre-northeast, far from the present-day Rangdan Empire on the galaxy's far western fringe. In fact, Old Rangda is closer to the galactic centre—the mighty supermassive black hole which every star in the galaxy circles around, like planets to a star—than Terra is. For unknown billions of years, the Rangdan's non-sapient ancestors preyed only on the dumb animals of their homeworld: the equivalents of Terra's deer, wolves, lions and gazelles. Eighteen-million years ago, on one of the most unfortunate days in the history of the galaxy, explorers of the Eldar came to Rangda. The Eldar were, and are, a species of mighty psykers, and they knew at once that this planet was home to no sapient life. Better yet, its atmosphere was breathable. It required very little terraforming to be converted to Eldar standards. Well-pleased with their discovery, the Eldar explorers settled down on this primitive world. And there it happened. One of the young Eldar, a scientist named Yleniad, strayed too far from his companions, overtaken by curiosity over an alien rock formation. He was a physicist, materials scientist and geologist, and the strange geology of an alien planet inspired, in him, an innocent interest.
Alas, as he was studying the strange-shaped rocks, Yleniad was taken by surprise by one of the local predators. Had this been any other predator, this would have been only a personal tragedy. It was not. It was a Rangdan Cerebravore. Worse, it was a female Rangdan Cerebravore, fresh from mating with a male. She had a newly fertilised egg which she was eager to deposit inside his brain.
When Yleniad woke up, bruised but seemingly unharmed, he did not know what had happened to him. Poor Yleniad was the Eldar equivalent of a human teenager, only two-thousand years old, out of a typical Eldar lifespan of sixteen-thousand. He could have consulted his friends and family, but he did not, fearing the embarrassment of having been ambushed by a non-sapient beast. He had survived anyway, miraculously, so what did it matter? Better to ignore the embarrassing incident and leave it in the past behind him. Yleniad's psychic powers told him nothing about his affliction. The Rangdan's effect on a person's body is purely physical. It does nothing to the soul. Perhaps because they originated as parasitic brainworms, the Rangdan are entirely soulless. They are not Pariahs, who create a field that suppresses Warp phenomena everywhere around them, but Blanks, who simply resist the Warp and are invisible to it. The Skullcracker devoured the knowledge in his brain, then she burst open his skull and fled. She, P'thul the Awakened, became the first sapient Rangdan.
Ever since the black day of Yleniad's demise, the Rangdan have been a sapient species with the knowledge of physics and materials science of the ancient Eldar. Luckily, much of the Eldar's technology depended on their psychic powers, which the Rangdan do not have. As such, the Rangdan could not simply duplicate all the Eldar's technology once P'thul the Awakened gained knowledge of it. However, they understood enough that they were able to achieve a rate of technological progress dazzlingly far beyond the rate of most species. P'thul the Awakened's children knew everything she knew, and their sapience was such an advantage that her descendants soon overtook the rest of the Rangdan. With her stolen knowledge, the Rangdan went from pre-Stone Age to spaceflight in less than a tenth of the time it took humans to do it. The first starships of mankind left Terra in the 15th millennium, and the first Warp-capable ones in the 18th millennium. The first Warp-capable Rangdan starships left R'angd a bare two-thousand years after P'thul's awakening.
There, in the stars, the Rangdan built a mighty empire for themselves. They defeated many other species with the mighty weapons they built using the technological knowledge they had stolen from the Eldar. Yet these species were not destroyed, as both the Eldar Empire and the Terran Federation did to their alien enemies. They suffered a fate far worse than mere genocide. They were enslaved, confined to their homeworlds, stripped of all technology that could give them spaceflight, and relentlessly preyed on by the Rangdan to use as Brain-Wombs: the Rangdan's term for the victims they use to reproduce. For the Rangdan prefer to use sapient Brain-Wombs. Dumb animals are sufficient to perpetuate the Rangdan species, but they like the uniqueness of experience they get from devouring new sapient minds. The Rangdan were careful not to kill too many of the inhabitants of the planets they conquered, lest they have no more to house the eggs of the next generation. Rather, they kept the local and Rangdan populations at a sustainable level.
Sapient beings were not being slaughtered for the sake of slaughter. They were being farmed.
The Rangdan Empire grew and grew and grew. Yet they were also wary. They knew very well that they were not alone among the stars, thanks to the knowledge of Yleniad the Devoured. The Eldar who had landed on Rangda had abandoned the planet, shocked and afraid when they saw their beloved son's skull burst open to reveal a monster underneath. But the mighty empire they had come from was still there. The Rangdan knew they must be wary of it.
Half a million years after Yleniad's demise, the growing Rangdan Empire encountered the rival that had given it birth. The resultant war was fierce from the start and grew yet fiercer once the Eldar understood its nature. While Rangdan are Blanks, immune to the touch of the Warp, they are not immune to the cruel and inventive tortures dreamt up by the wickedest minds of the Eldar. Through the torture of captured prisoners, the Eldar came to understand how the Rangdan had acquired the great knowledge of science and technology that they possessed. The murder of Yleniad the Devoured sparked rage amongst the Eldar, not so much for the brutality of it (the Eldar were no strangers to brutality themselves) as the indignity. The Eldar perceived themselves as the heirs of the Old Ones, rightful lords of the galaxy, the highest and best species in the universe. For some lesser species to consume one of their own was beyond an injury. It was an insult that must be answered in fire and blood.
The Eldar named the Rangdan the Dulaithiel, which means Usurpers-of-Thought in the Eldar tongue, and they waged war against them with terrible fury. The robotic automatons, Wraithbone starships and psychic armies of the Eldar shattered the hosts of the Rangdan Cerebravores. The war lasted for sixty-thousand years—quite impressively long, by the standards of the Eldar Empire's wars after the War in Heaven. The Rangdan were a mighty opponent, perhaps the mightiest faced by the Eldar Empire since the War in Heaven ended. The outcome was inevitable. The Children of Isha triumphed over all comers. The Rangdan Empire was smashed, its worlds conquered, its mighty starships blasted from the sky, its population slaughtered, its subject species exterminated. Even R'angd itself was laid to waste. It became an Eldar world, on which not a single Rangdan has set foot ever since. Quadrillions of Rangdan—men, women, children fresh from their hatching—were slaughtered in cold blood by the dread hosts of the Eldar. None must dispute the right of the Aeldari to be masters of the galaxy.
Scattered starships of survivors fled from the twitching corpse of the Rangdan Empire. The Eldar pursued them, these Dulaithiel, with hatred and rage. They intended that not a single Dulaithié should survive their empire's downfall. Whole planets, colonised by just a few Rangdan refugees, had their crusts melted down to the mantle. Millions of starships were hunted down and blasted from the void. Nearly every Rangdan in the universe perished.
…Nearly every.
Just one starship of Rangdan refugees survived the Eldar's fury. The starship itself did not survive. It was slashed to ribbons by Eldar particle-beam weaponry. Its main reactor exploded in a fiery thermonuclear blast and fragments of wreckage crashed down on a desolate, bare rock of a rogue planet in the far west of the galaxy. The Eldar starship's shipmaster, a young woman named Arilyn, a daughter of the Eldar high nobility, thought her foe destroyed utterly, and she departed to hunt down more Rangdan elsewhere. She had not counted on the extraordinary toughness and resilience of the Rangdan. Out of the thousands who had been on-board the ship, mere dozens of Rangdan crawled out of the wreckage. They were wounded by shrapnel and the force of impact, blasted by radiation, and yet they were alive.
The Rangdan survivors found life on this deathly cold rogue planet floating in the void of interstellar space. This life was mere rodent-like creatures, the size of Terran rats, scurrying underground, warmed by the rogue planet's core, where the absence of a star was not felt so keenly. Their small minds were hardly satisfying prey to the Rangdan's sapient-preferring appetites. For species survival, though, they were good enough. The Rangdan built a new civilisation for themselves, far underground, on this rogue planet in the galactic far west. It took aeons for them to multiply to anywhere near their previous level. Worse, inbreeding—for there were, at first, a mere thirty-seven Rangdan left—damaged the genetic memory that gave them flawless knowledge of everything their mothers and their mothers' mothers had known. No longer did the Dulaithiel perfectly remember the knowledge of physics and materials science of the Eldar Empire that had been passed down through the generations since P'thul the Awakened and Yleniad the Devoured. They still remembered much of it, but their memory was imperfect, incomplete, corrupted by the genetic malfunctions due to inbreeding.
Still, in spite of everything, they were alive. And they meant to keep it that way. Over millions of years, the Rangdan rebuilt their civilisation. In time they regained the industrial base required for spaceflight. They expanded beyond T'Chikh to other worlds. T'Chikh was the name they gave the cold, unlit rogue planet that was their new homeworld, a poetic name from the Rangdan language which translates as somewhere between 'exile' and 'rebirth'; the easiest human-analogue word would be 'wind-scattered seed which will bring forth new growth'. And grow, indeed, they did.
The First Rangdan Empire was no more. The Second Rangdan Empire was in the making.
In battle, the armies of the Rangdan are fearsome foes. Every adult Rangdan is extremely resilient. In sheer dogged toughness to survive, they put every other known sapient species to shame, from humans to Orks. They cannot be decapitated, for they have no heads. They can survive losing limbs, being blasted with radiation, and absurd amounts of blood loss. Their natural, reptile-like armour of black scales is extremely strong. Bullets—the non-exploding projectiles fired by primitive weapons, known as stubbers, which are rarely used by the Imperium—will simply bounce off their scales. Bolt shells, grenades and antitank guns will hurt them, but are unlikely to kill them (though not impossible). The only ranged weapon that will definitely kill an adult Rangdan Cerebravore is a direct hit from artillery or the main gun of a battle tank, or the enormous weapons carried on Titans. The Rangdan's black scales are made of superb insulating material, excellent at dispersing heat, and so they are virtually invulnerable to plasma and laser weapons, or any other weapons which rely on heat energy. Trying to kill a Rangdan with heat is folly. Many a soldier has seen a Rangdan disappear in a huge, fiery explosion, sighed with relief, and then watched with horror as the Rangdan strides out of the inferno, unharmed and undaunted.
The only truly reliable way to kill a Rangdan is to strike his or her brain—which is buried deep within his or her heavily armoured torso—with a high-momentum blow like a melee weapon, not a high-pressure low-momentum blow, as with a bullet or a bolt. This blow must be aimed directly at the brain, in the centre of the torso. It is almost always foolish to try to attack a Rangdan from range. They are better confronted in melee: a stab from a chainsword or a crushing blow from a Power Mace strong enough to break through the scale armour and aimed at the centre of the chest.
With this strength, the Rangdan can fight on battlefields against the Emperor's Space Marines. Those Imperial soldiers who are not Astartes would find it very difficult to stand a chance. Rangdan Cerebravores are a lot stronger than ordinary humans. They are also a lot rarer than ordinary humans. Because the Rangdan Empire spent aeons clinging to cold inhospitable worlds on the far fringe of the galaxy, living off rodents scurrying underground, its population is small compared to the teeming multitudes of mankind. The Rangdan's numbers have been expanding rapidly since the Fall of the Eldar, but expanding from a very low starting point.
As for Astartes, the Rangdan's natural scale armour is inferior, in resistance to melee weapons, to the ceramite Power Armour that is worn by the Emperor's Space Marines. The Rangdan are much superior, however, in resistance to ranged weapons, because of their superb tolerance of heat and their extraordinary resilience against any blow that does not reach the brain. They are less heavily armoured than Astartes and weaker in physical strength. As such, in close combat, a Rangdan is somewhat inferior to a Space Marine one-to-one, as long as the Space Marine is wearing Power Armour. (If the Space Marine is unarmoured, the Rangdan would win easily. The Space Marine would stand no chance at all.) They are not enormously inferior, however, and there are ten times as many of them as the Space Marines' numbers.
In combat, the Rangdan make great use of the fact that every Rangdan has four arms and two legs. Despite their far-from-human appearance, they are not dumb animals. They are tool-users every bit as intelligent as humans. They do not fight with tooth and claw, unless it is a lone Rangdan who has grown up from a Skullcracker hatched on-board a human starship. The armies of the Rangdan Empire wield guns and swords in their four arms, especially guns. Their guns bear a noticeable resemblance to Eldar design and are as advanced as one would expect from that comparison, firing Eldar-like projectiles with deadly sharp, monomolecular edges, or bursts of laser-light or plasma. (Just because the Rangdan themselves are not vulnerable to such weapons, does not mean they do not see them as valuable weapons to use against other species.) As a general rule, the Rangdan prefer to engage Space Marines at range, where their lethal Eldar-made guns can take best effect. The Space Marines prefer to close the distance and cut the Rangdan down.
In void warfare, the Rangdan's starships make great use of the knowledge they stole from Yleniad the Devoured of the ancient Eldar's technology of the material universe. Due to the years of inbreeding between the mere thirty-seven individuals who re-established the Rangdan race after the First Rangdan Empire fell, their genetic memory has been damaged, so they do not perfectly remember the technology of the ancient Eldar. They do remember enough that their starships are vastly superior to the Imperium's on a tonne-for-tonne basis—even superior to the best that could be accomplished by mankind in the Golden Age of Technology. However, the Rangdan's war-fleets are also far outnumbered by the Imperium's. The Rangdan were living in exile and obscurity for millions of years, clinging to the galactic edge, ever fearing to catch the eyes of the Eldar, the ancient enemy, and thus provoke the renewal of the extermination campaign. In that time, they could and did multiply, but at not nearly as fast a rate as they have done since they learnt that the Eldar Empire fell. And as it would be obviously foolish for the Rangdan to trust their oppressed captive species to labour in weapons manufactora, their low population numbers limit their industrial base. The Rangdan can produce far better starships than the Imperium can, but they simply can't produce enough of them to match the star-fleets of the Imperial Army and the Legiones Astartes in numbers of starships. Nor do they match Imperial starships in size. The Rangdan have never been seen to operate any starship longer than two kilometres. Most are about one kilometre long. For contrast, the Imperium would not consider any starship less than eight kilometres long to be worthy of being considered a starship of the Wall-of-Battle, a 'battleship'. And the Imperium's mightiest starships, the great Gloriana-class battleships, are each twenty-six kilometres long. And due to simple arithmetic, if one starship is ten times the size of another in each dimension, it has a hundred times as much surface area for guns batteries, and a thousand times as much volume to store reactor fuel, shield generator fuel, and reserves of ammunition.
As well as true Rangdan, the Rangdan Empire also uses armies of slave cultists recruited from its Womb-Worlds. These poor souls have been taught to believe that their alien masters are gods and that there is no higher honour than to serve them. All have lived for their whole lives on worlds where the Rangdan Empire has ruled since before they were born. Despite the Rangdan's best efforts at indoctrination, they have not managed to persuade their cultists that the horrific fate of conversion into a Brain-Womb is a divine reward. Eventually they gave up trying. Cerebravore-worshippers still fear this fate, and it is used as a punishment for those deemed to have failed in the worst ways, such as deserters who flee from the battlefield. Because of their soulless and obscene nature, the Rangdan find it difficult to understand the psychology of humans and other species with souls, so their brainwashing is not particularly effective. Most Cerebravore-worshippers serve the "black gods" more out of fear than true loyalty. They do not rebel because they are more afraid of their gods than their enemies. Discipline is ruthlessly enforced, invasive implants apply electric shocks to their bodies if they hesitate or disobey, and any hint of mutiny is greeted with brutal retaliation. These armies of Cerebravore-worshippers are poorly armed, because the Rangdan, quite sensibly, do not trust their slave species not to turn against them if given the sort of weapons that could give them a decent chance of winning if they did. Cerebravore-worshippers tend to use 'human wave' attacks, which is not to say that they are all human: Cerebravore-worship cultists are a mixture of about 30% humans and 70% countless xenos species of every shape and form imaginable. Despite the failings of Cerebravore-worshippers, because of their sheer numbers they remain a threat to the Imperium. They are also a potent sight to stir the spirits of the Imperial Army or Planetary Defence Forces. The pitiable sight of wild-eyed humans throwing their own lives away as slaves to xenos "gods" is a reminder of the virtue of what the Imperium fights for: the Imperial Truth, death to xenos, and human unity.
As long as the Eldar Empire yet lived, the Second Rangdan Empire kept to the galaxy's far western fringe, colonising a long line of star-systems northward and southward. The Rangdan did not underestimate their foes. They took great care not venture near the ancient enemy. Despite their preference for sapient minds, they often colonised the cold moons of gas giant planets further from their stars than Saturn is from Sol, or rogue planets, which are even colder. They had to subsist on rodent-like creatures which spent their lives far underground, warmed by the geothermal heat rising up from the mantle. The Rangdan would have preferred warmer planets. Their favoured temperature is actually somewhat warmer than mankind's, as is known from the fact that their starships have hot, humid, slimy interiors. But they knew the necessity of avoiding the attention of the Eldar. If the Children of Isha learnt that a single Dulaithié still breathed, their empire would surely come down on the Dulaithiel with all of its terrible power.
Of course, the Rangdan did not forget the genocide the Eldar wrought against them. They swore vengeance on their old tormentors. Once, the Eldar's hatred of the Dulaithiel was one-sided. Now, since the genocide, the hatred is reciprocated in full. The Rangdan hate the Eldar like nothing else in the universe. Whenever they receive news of an Eldar Craftworld, a planet of Exodites or a Dark Eldar pirate base, they will actively go out of their way to see it burn.
Before the Age of Strife, the Rangdan had no organised contact with mankind, as the sons and daughters of Terra spread out eagerly in the galaxy's more central regions. A few of the more adventurous colonists went to explore the furthest of the far galactic west and disappeared, never to be seen again. The Terran Federation never found out why. There were whispers that there must be a mysterious peril in the furthest of the far galactic west. No human alive knew the truth of it.
Thus they might have remained, unaware and uncaring of mankind, were it not for events far, far away.
On distant Terra, tens of thousands of light-years away from T'Chikh and the Second Rangdan Empire centred upon it, the wandering immortal scientist who had in recent centuries adopted the mantle 'Emperor of Mankind' was working on the extraordinary work of genetics that would become the Primarchs in his Himalazian laboratories. He did not work alone. His was of course the greatest mind in the project, the leading one, and the source of many fabulous insights. But he knew the folly of attempting such a vast project single-handed. Other scientists, those few who were both knowledgeable enough to assist him and trustworthy enough in their beliefs in a better future for mankind, assisted the Emperor in the great effort of the Primarchs' creation.
One of the most senior of these scientists, a woman named Erda Broutik, perhaps the most brilliant scientific genius of her era, came to disbelieve in the righteousness of the Emperor's hopes and dreams for mankind. She concluded that her lead researcher was a villainous tyrant. His plans to unite the species were madness and hubris. His declared intention to pass power to a Council of Terra and step back into his current seclusion must be lies. Obviously he was a monster who desired nothing more than to enslave mankind.
With these thoughts in her mind—unprobed by the Emperor, out of a courtesy to his fellow researchers, a kindness which Erda gave him cause to bitterly regret—Erda set about preparing the undoing of the Emperor's works. Through her technically brilliant mind, she devised a subtle, careful sabotage of the wards that protected the Emperor's laboratories from external Warp influence. Doubtless she thought she was being clever. She was not. She planned to have the Primarchs teleported away, removed from their father, so that he could not use them to bring about his mad dreams of galactic unity and conquest. In a way, she got what she wanted. The Primarchs were taken away… but not by her. In her hubris, Erda Broutik had not understood that there were forces at play other than herself and the Emperor. By sabotaging the wards, she had left the Primarchs open to something else.
Nobody knows who, exactly, came into the Emperor's laboratories from the Warp and took away the infant Primarchs, except the master of mankind and his two closest confidants Constantin Valdor and Malcador the Sigillite. The Emperor has been understandably tight-lipped on the subject of the tragic loss of his daughters and sons. It is known, however, that someone did. And it is known that Erda Broutik's treachery was the reason why this incursion from the Warp was possible.
When she realised that her plans had gone awry, Erda fled from Terra, sneaking out on one of the small starships that had been meant as a scout-ship for the Great Crusade. The Navigator, Darius al-Thusi, was a man loyal to the Emperor, whom the Navigators call the 'Lord of Light', for he is the golden beacon they follow when they need to find the safety of the Materium from deep inside the realm of madness of the Warp. He was no traitor. But Erda deceived him. He did not yet know of her treachery. All he knew was that Erda Broutik, a highly trusted woman in the Emperor's inner circle, was demanding immediate passage offworld. Loyal to the last, he obeyed her.
Like his master, Darius al-Thusi would have cause to regret his trust in Erda.
The treacherous scientist knew that the galactic east extends further from Terra than any other direction in the galactic plane, because the Sol system itself lies to the west of the supermassive black hole at the centre of the galaxy. The Emperor would know that she intended to go as far from him as possible. Therefore, she reasoned, she would be expected to go east. So she went west. Her starship travelled as far west as she could. It dropped out of Warp at the furthest western edge of the galaxy, where the fields of stars faded away into the unfathomable intergalactic void.
That was where she was found by the Rangdan Cerebravores.
Their stolen starship was boarded. Both Erda Broutik and Darius al-Thusi were assailed and became Brain-Wombs for Rangdan Skullcrackers. This bequeathed unto the Rangdan the critical piece of information which they had lacked until now: the Emperor's knowledge that the Eldar Empire had just Fallen. The galaxy was ripe for the taking.
This news sparked a feeding frenzy among the Rangdan Cerebravores. At a stroke, the Second Rangdan Empire abandoned the caution it had held for millions of years. No longer did the Rangdan confine themselves to the least hospitable places in the galaxy: rogue planets and far-out moons on the galactic western edge. They surged outward, conquering many inhabited worlds in the galactic west, some human but most belonging to a variety of xenos species. Each conquered planet became a Womb-World, its people deprived of spaceflight technology and enslaved to be eaten or to serve as Brain-Wombs for the vile process of Rangdan reproduction. (In addition to the obscene hatching of Skullcrackers, the Rangdan are carnivores. They eat only meat and they prefer it to be the meat of sapient prey.) Over the years, quadrillions of souls owe their misery to Erda Broutik.
Because of the way in which they became aware of events in the outside galaxy, the Rangdan are well aware of the threat of the Imperium, the Emperor and his intent to conquer the galaxy. The Imperium, meanwhile, is entirely unaware of them.
For interstellar travel, the Rangdan, being Blanks, do not understand the Warp, but they know that they do not understand it. For a long time, they relied on starships which took short, shallow jumps into the Warp, never venturing deeper than the surface between Materium and Immaterium. (This method, indeed, was used by every species that travelled faster than light, except the Eldar with their Webway.) This greatly limited their speed compared to Imperial starships. Since they found Darius al-Thusi, however, they have known of the concept of Navigators. Now, they use them. They process conquered human populations and identify those few with enough psychic sensitivity that they can feel the Emperor's light, the Astronomican that shines across the stars. These proto-Navigators are brutally tortured until their minds break, subjugated with chemical control and electrical brain manipulation, and used to pilot starships across the stars. It is cruel, but it works well enough. These traumatised souls are still able to see the Emperor's light clearly enough to follow it when they are prodded and told to lead the way out of the Warp. This process is so brutal that the Rangdan's Navigators rarely live more than a couple of years of service, unlike in the Imperium, where Navigators live long enough to have children.
This forces the Rangdan Empire onto a path of violent military expansion. If they ever ran out of newly conquered human planets, they would run out of new Navigators and they would be forced to shift to a much slower form of Warp Drive, or else their empire would crumble.
Worse, the mere fact of the Imperium's existence is not all that the Rangdan gained from Erda Broutik. The Skullcracker who burst forth from the head of poor, loyal Darius al-Thusi was unremarkable. The Skullcracker who came from Erda's was not. She, R'zun, was a Rangdan with all the knowledge of one of the best geneticists in the history of the human species, one who had worked personally alongside the Emperor in the project of the Primarchs' creation.
R'zun grew up to be a great geneticist, and she spent years, then decades, experimenting with what she had learnt from the Primarch project. Countless attempts were made and failed. It is true that the Rangdan genome is very different from mankind's, but that was not the main reason. The Primarchs are creatures of the Warp, each and every one, even those who never use any active psychic powers. Their souls are every bit as enhanced and artificial as their bodies, created with care by the Emperor. Genetic and psychic enhancement, melded together and acting in perfect harmony. The genetic enhancement of the Primarch project was done by the Emperor and his fellow scientists, including Erda. The psychic enhancement was done by the Emperor alone. As the Rangdan are Blanks, blind to the Warp and invisible to it, the psychic elements of Primarch creation cannot apply to them. Erda Broutik had been a genius, however, and R'zun inherited that brilliance in full. Eventually, she built the first of her 'Glorious Children': vile creatures known to the Imperium as Xenarchs.
A Xenarch stands five metres tall, taller than any Primarch, and is covered from head to toe in black scales tougher than normal Rangdan scales, even tougher than Power Armour. They move shockingly fast for creatures of such size, as is also true of the Primarchs. They are also superhumanly strong and intelligent. A single Xenarch will cut through armies of Astartes like a scythe, dealing death wherever she goes. Only the Emperor and the Primarchs can fight them and have any chance of victory.
Xenarchs are untouched by the Warp, like any other Rangdan Cerebravore, and so they are not an exact analogy to Primarchs. They lack the perfect melding of genetic and psychic enhancement. Because of that, they are not nearly as far exalted above the Rangdan norm as Primarchs are above the human norm. However, an unmodified Rangdan was far more powerful in combat than an unmodified human to start with, so it balances out. Overall, a Rangdan Xenarch is every bit a match for a Primarch of the Imperium of Man. And R'zun made thirteen of them.
Why thirteen? Why not less, or more? Some in the Imperium believe that R'zun feared making too many Xenarchs, lest they challenge her power. Others suggest that she ran into the same mysterious limitation that prevented the Emperor from making a whole army of Primarchs to save mankind. None but R'zun and the Emperor know the truth of it.
In any case, with the thirteen Xenarchs, R'zun soon seized power in the Rangdan Empire. The old aristocratic authorities could not stand up to the Xenarchs' might. She has, since, inspired a personality cult of extreme depth. The Rangdan people are atheistic, or so they claim, but they practically worship her as a living goddess. They call her the Mother-of-All, they speak of her every day before they eat a meal, and they believe that her sight spans the galaxy and can look upon them when they do battle in her name. Propaganda has even begun to conflate R'zun with P'thul the Awakened, carefully shrinking the distinction between the Mother-of-All and the literal mother of the Rangdan as a sapient species.
For centuries, this Third Rangdan Empire has been preparing for war against the Imperium. In a violent frenzy of expansion, the Rangdan have seized hundreds of thousands of human worlds. Now they stand poised to fight the war they have long expected. These planets have been transformed into Womb-Worlds. The fate suffered by their human peoples is not as pleasant as mere slaughter. Those suffering trillions have been penned in like animals, deprived of the technology for spaceflight, and left to spend the rest of eternity as a source of meat for Rangdan tables and Brain-Wombs for the obscene, unholy process of Rangdan reproduction. The Rangdan are careful not to wipe them out or kill too many such as would risk that outcome, lest they be denied their supply of food and of new Brain-Wombs. Their victims are not granted even the mercy of extinction.
If the Rangdan triumph, this grim fate will befall all of mankind. Every Imperial planet will become a Womb-World. Mankind's teeming quadrillions will live as slaves, either to be messily murdered for the feeding and spawning of their alien overlords, or to breed their sons and daughters to be offered up for that fate.
On the one side, stands the Imperium of Man: the Emperor of Mankind, his Primarch sons and daughters, his Legiones Astartes, his Imperial Army, his Titan Legions and all who serve mankind in his name. On the other side, stands the Rangdan Empire: hordes and hordes of hideous black-armoured behemoths, each one a born predator, a born killer, waiting to be unleashed against their prey. The Xenarchs stand above them, highest avatars of the Mother-of-All's will. And at the apex of the Rangdan Empire, R'zun still lives and rules. The average lifespan of a Rangdan is one-hundred and fifty years. R'zun has lived far beyond that, preserving her life through processes of obscene genetic modification. Her body is little more than a shell, clinging to life like a parasite on the fabric of the universe, kept alive by a complex array of biochemical interventions overseen at every instant by a thousand constantly working super-cogitators. Her mind, however, remains every bit as formidable as it was when she broke the secrets of the Primarch project and created the Xenarchs. She is mother, queen and living goddess to the Rangdan Empire. Only a fool would fail to fear the goddess of the black gods, and fear what new monstrosities and cruelties she will devise and unleash against the ever-suffering humans of the galaxy.