"Fear The Alien. Hate The Alien. Kill The Alien."
- — The Imperial Infantryman's Uplifting Primer (Damocles Gulf Edition)
Xenos (pronounced ZEE-nos or zeh-NAHS) is a High Gothic term used by the people and adepta of the Imperium of Man that means "stranger" and "alien." It is often used by the Adeptus Terra to signify enemy aliens or off-world, non-Human forces.
The main starfaring, intelligent xenos species in the Milky Way Galaxy are the Drukhari, Aeldari, Necrons, Orks, T'au and Tyranids, though there are countless other intelligent alien species, most of them hostile, that Mankind has encountered over the long millennia of its expansion across the stars.
Xenos Interactions with Humanity
"Only a madman deals with the alien. Only a fool relies on the alien. Only the dead have trusted the alien, although the last may be deluded into claiming that they yet live."
- — Lord Militant Tarkus Hol in his memoirs, Observations on the Inevitability of War
It is not recorded exactly when the human race learned that it was not alone in the galaxy, or which intelligent alien species it first made contact with, but what is known in early Imperial records is that humanity has been competing with xenos since the Dark Age of Technology. The first human interstellar civilisation which existed at that time made contact with many alien races, possibly including the Aeldari and the Orks, but the majority must have been hostile or incapable of co-habitating with humans as no aliens are known to have ever willingly integrated into the broader interstellar human civilisation of this time.
When the Age of Strife began with the collapse of the first human interstellar federation, many suddenly isolated human colonies were set upon by all manner of hostile aliens, though a few notable exceptions of peaceful coexistence occurred as well. The scattered outposts of Mankind would be preyed upon for millennia by various xenos species, pushing the human race towards eventual extinction until the Emperor of Mankind revealed his existence and began his galaxy-spanning Great Crusade to reunite humanity under a single interstellar government. The Emperor believed that only in unity -- unity bought at nearly any price -- could Mankind survive a galaxy that was far more dangerous than any human savants of an earlier age would have believed.
At its inception, the Imperium of Man made one of its core objectives the extermination of all alien life forms that posed any threat to humanity. Thus many species suffered extinction at the hands of the newborn Imperium which perpetrated one act of genocide after another. This would also apply to the majority of human civilisations encountered during the course of the Great Crusade like the Interex and Diasporex civilisations, which had founded cultures where alien and human populations had successfully integrated into a single whole. Diplomacy was rarely tried, and was more often offered only to other human civilisations encountered in the course of the Great Crusade. Certain aliens, such as the Eldar, who were more technologically advanced than the Imperium were often simply left alone so long as they did not threaten human interests.
The Imperium of Man's extreme xenophobia and outright call for genocide against all other intelligent species in the galaxy is in some ways justified by the sheer hopeless belligerence of most xenos races encountered since the Great Crusade. Almost all alien races encountered by Mankind since the Emperor first began to push his forces out amongst the stars have been so malevolent or aggressive toward humanity that any form of negotiation or parley often proved impossible. Other instances, such as during the Imperium's first encounter with the human-alien alliance known as the Diasporex, would end in violence because of the inherent distrust the Imperium had for all alien civilisations even if there was common ground that could be found.
This policy of unconditional hatred persists into the modern Imperium more than 10,000 standard years after the Great Crusade ended in the blood and fire of the Horus Heresy. In that time, many more alien civilisations have been discovered and destroyed regardless of threat level, culture, disposition, or technology. These sentiments of xenophobia have been reinforced by the Imperial Creed, which preaches that Mankind has a divinely-inspired manifest destiny to be the sole ruler and inhabitant of the galaxy. Humanity alone, the Imperial Creed teaches, is the sacred form of intelligent life chosen by the God-Emperor to spread out amongst the stars. All other intelligent life is to be reviled as an abomination and an affront to the God-Emperor's plan for Mankind.
Much of the Imperium, led by the religious fervour of the Adeptus Ministorum, believe in the extermination of all aliens, even those that are entirely peaceful species. When the masses are stirred to such fear and fanaticism, they demand destruction over any kind of cooperation or coexistence. This might seem like a harsh measure, but it is an age where ignorance is a virtue and brute force is used to solve a multitude of problems; the dangers of failing to act are too unthinkable to elicit any other response. It has not become this way without reason; most aliens encountered by Mankind are hostile and devious, and some have proven as ruthless and ambitious as humanity itself. To them, there is no room in the galaxy for the Imperium.
Despite the many xenos dangers across the Imperium, there are those who, in their ignorance or desperation, are willing to make pacts with aliens. Sometimes this is seemingly innocuous: trading to gain advanced technology, hiring brute labour or buying mercenary help. So, concealed within teeming Hive Worlds, xeno-tech draws high prices in illicit markets, while the carnivorous Kroot have been paid in flesh to fight alongside human Planetary Defence Forces.
In the Tollovian Cluster, fickle Caradochians are not only tolerated, but allowed to sell their military services to the highest bidder. In this way, many argue, humanity is tainted. After all, accepting a Donarathi amongst our own kind is only a small step away from making open covenant with the Hrud, the Ork, or any of the other foul alien species that plague the galaxy.
In the broadest of terms, no human may interact with a xenos creature in any way except violently—such creatures are inherently blasphemous against the God-Emperor and must be eliminated on sight without exception. From a young age, citizens of the Imperium are taught to fear and hate the xenos, and innumerable legions of Imperial Guardsmen are taught of the superiority of Man over all inhuman things as they are drilled in the use of Lasgun and bayonet. The number of xenos who have set foot upon human worlds peacefully and survived is small indeed.
But this is not always the case. Though the Imperium was founded upon a creed of intolerance towards non-humans, peaceful contact with a variety of xenos species actually happens frequently. In most instances, these are short-term alliances or matters of temporary mutual convenience. Just as frequently, illegal contact with xenos cultures is sufficiently commonplace that the Inquisition and the Adeptus Arbites keep a constant watch for potential contamination through contact with xenos; in spite of such vigilance, the “Cold Trade” of xenos goods on the black market still exists. A number of xenos species hire themselves out as mercenaries to a number of other species, for reasons all their own, and there are more than a few unscrupulous Imperial Commanders willing to hire such creatures to bolster their forces, seeing them as the ultimate expendable mercenaries and caring little for their fate.
Rogue Traders, and a few other individuals, have the power to do more, and do so legitimately. These men and women, freed from the normal strictures and feudal requirements of the Imperium, have the influence and authority to make peaceful contact with xenos species for trade, temporary alliance or even to hire them as mercenaries. In some cases, an allied xenos may almost become a trusted companion, highly valued for its skills. Individuals with trusted xenos allies may find that the normal laws and doctrines against consorting with xenos hamper their operations, and in regions on the fringes of the Imperium, like the Calixis Sector, a method of sanctioning xenos allies has come about to ease these difficulties.
These Sanctioned Xenos are still regarded with caution and hostility by most, but this is unsurprising -- even the Sanctioned Psykers of the Imperium are still subject to the prejudice against witches that is so readily promoted in the Imperium. However, so long as they remain in the company of an ally who can vouch for their service (assorted documentation in copious amounts is provided for this purpose), and agree to be visibly marked to prove that they are Sanctioned -- this mark is typically a brand or tattoo on the face or hands, where it can be clearly seen -- they may continue existing. The permanence of such a mark is of no concern -- without a human employer to vouch for the xenos, the mark is essentially meaningless, and should the xenos cease to serve the Imperium, its life is forfeit anyway, and the creature is slain as soon as possible.
Within the Calixis Sector, obtaining such Sanctioning is an expensive and challenging matter; there are few who will even concede that such a practice exists, let alone perform it. A handful of Administratum Ordinates in Port Wander provide the service, having been recruited for such a purpose by interested Inquisitors, whose purity is tested on a regular basis to monitor potential contamination. Their identities are not widely known, and the service itself must be specifically requested, as they will not offer it otherwise.
As rare as Sanctioning is, it is recognised in many of the more well-travelled parts of the Calixis Sector and the settled worlds of the Koronus Expanse. As the required mark is very similar to that which identifies Sanctioned Psykers, it quickly becomes apparent what the mark means to those with any knowledge of the structure of the Imperium. It is still wise to be cautious in bringing even a Sanctioned Xenos to a human world; those who will understand the nature of the mark and the notion of Sanctioning are still a minority, and a xenos creature will spread only fear and anger by its presence amongst the common masses of mankind.
The Manifold Dangers of the Alien
Although the potential threats posed to humanity and the Imperium by alien influences are many and varied, the doctrine of the Ordo Xenos defines five broad avenues by which the xeno can afflict mankind:
- The Threat of Discord - Perhaps the most insidious threat the alien can pose to the Imperium is through the deviancy of its ways and ideas, exposure to which might inspire envy, apostasy, or even adoration, leading humanity away from the Imperial fold and into heresy and rebellion. The Imperial Creed is the greatest weapon against such wrong-thinking. But the further from the centres of civilization one goes, the more the risk of mental and moral contamination increases.
- The Threat of Temptation - The alien is a thing of lies and false promises, offering gifts and trinkets, powerful technology, or alliances -- all of which are ready lures for the weak-minded, avaricious, or infirm of faith. There is a shameful roll call of traitors and deviants who have succumbed to such temptations in the past. The dubious treasures and lost secrets of xeno-civilizations long extinct can hold equal threat for those who disturb their resting places.
- The Threat of Corruption - The alien can pose a threat of bodily taint and corruption just as easily as the powers of the warp. This taint may take the form of parasitism for the purpose of vile promulgation of its own species or to appease its hunger, such as with the Lacrymole. Alternately there is the crime of transgenic blasphemy; the deliberate combination of human and nonhuman DNA or tissue falls into this category.
- The Threat of Dominion - Some xeno-forms have dark powers to dominate and control humanity by psychic or other means, perhaps even forcing others to worship them as false gods. Abhorred creatures such as the Nostrafex, the Simulacra, and the Cryptos pose insidious and subtle dangers, turning humans against their own and treating them as little more than cattle or playthings.
- The Threat of Destruction - Some aliens seek to invade, conquer, and destroy by overt means, from the murderous swathes cut by great Ork Waaaghs to the invasions of the enigmatic Thyrrus. Likewise, some non-sentient species, be they dangerous predators like the feared Void-Cacaradon or ecology-destroying vermin such as the Xothic Blood Locust, are simply too hazardous to be allowed to live whenever encountered.
Major Xenos Races
Though there have been hundreds of sentient species in the galaxy's long history, only a handful have ever managed to build vast interstellar empires beyond their respective homeworlds. Even fewer have ever been powerful enough to rule the galaxy for any length of time. While Mankind is the current dominant species of the Milky Way Galaxy, it was not the first, and it is constantly challenged from without by rival species who envy its position of power. What distinguishes these races from the myriad of lesser intelligent species is the fact they are all currently too powerful for the Imperium of Man to exterminate like so many others:
- Aeldari - The Aeldari, also called Eldar by some, are an ancient, but dying humanoid race whose technology remains far more advanced than that of the Imperium. The Aeldari once possessed a galaxy-spanning empire that exceeded even the Imperium of Man in size and strength. After the Fall of the Eldar shattered their civilisation, the Aeldari were reduced to a refugee population as the majority of the race now exists in worldships known as Craftworlds drifting across the galaxy. These Aeldari refer to themselves as Asuryani. The Aeldari hold humanity, or the "Mon-keigh" as they refer to humans, in disdain as it was the death of their empire that allowed Mankind to rise during the Great Crusade, yet they also assist the Imperium against common foes like Chaos and the Tyranids whenever it suits their own purposes.
- Drukhari - An off-shoot of the Aeldari race with its origins before The Fall sometimes inaccurately called the "Dark Eldar," the Drukhari differ radically in culture if not biology from their Craftworld Asuryani cousins. Centred around the Dark City of Commorragh which is located within the ancient Labyrinthine Dimension of the Webway, the Drukhari are a depraved society characterised by sadistic violence and hedonistic excess who continue to indulge in the same vices that caused the Aeldari Empire's Fall. They are sustained by absorbing the psychic suffering of their victims and slaves, who they acquire from all the other intelligent species of the galaxy during raids from the Webway into realspace. Humanity often fails to make a distinction between the Asuryani of the Craftworlds and the Drukhari as the two can appear anatomically and technologically identical, though the cultural distance between them is actually vast.
- Necrons - The Necrons are a race so old that even the Aeldari would consider them ancient, possessed of technologies so advanced they defy the comprehension of the Imperium's best savants. The Necrons have lain dormant deep beneath their Tomb Worlds for millions of Terran years. They have only begun to reawaken in the last few centuries of the 41st Millennium. Cybernetic and virtually indestructible, Necrons have been emerging from their ancient Tomb Worlds across the galaxy to slaughter and kill the unsuspecting denizens of the galaxy, determined to reestablish their ancient galactic empire and eliminate all of the lesser races that have taken their rightful place.
- Orks - The Orks are a hyper-violent species of hulking green-skinned humanoids who have existed as long as the Aeldari, yet continue to be a powerful force in the galaxy due to the durability of their own biology, if not their civilisations. Universally perceived as a crude and savage species by all of the other intelligent races of the galaxy, this is for the most part an accurate assessment. Often fighting for the sheer sake of the joy they find in violence, the Orks are in a constant state of war with the other races of the galaxy and themselves. Due to the peculiar adaptability and efficiency of their reproductive spores, Orks may be the only species in the galaxy truly immune to extinction, and are more akin to an infestation than a species.
- T'au - The T'au are a young race of alien humanoids from the Eastern Fringe of the galaxy who have created a rapidly expanding interstellar empire with highly advanced technology in only a few Terran centuries. Led by the mysterious Ethereal Caste, the T'au are compelled by their pseudo-religious philosophy known as the Greater Good to expand their borders until they have assimilated and imposed their collectivist ideology upon the entire galaxy. Unique amongst the other major intelligent races of the Milky Way, the T'au Empire effectively employs a combination of diplomacy and measured military force to acquire other intelligent species as allies to their cause. Though barely a blip on the galactic map when compared to the vast interstellar territories that encompass the Imperium, the T'au Empire poses a realistic threat to seize the frontier of space away from Mankind if something is not done to counter their continued expansion. Even many human worlds of the Imperium have found the Greater Good an attractive enough philosophy to turn their back upon the dictates of the God-Emperor.
- Tyranids - The Tyranid species -- actually a collection of different bioforms all genetically engineered to serve specific functions by a psychically gestalt Hive Mind -- currently presents the greatest threat to the Imperium and every other intelligent race of the galaxy. The Tyranid menace comes from the great void beyond the boundaries of galactic space to strip all the life-sustaining planets of the Milky Way Galaxy of their precious bio-mass in a quest to constantly improve and evolve their species towards new heights. Ever evolving and learning to adapt to the tactics of each new foe, no less than nine Tyranid Hive Fleets have been identified by the Imperium of Man entering the galaxy. Several of these Hive Fleets have yet to be destroyed or contained, while others have been shattered through immense expenditure of lives and resources, only to see new Hive Fleets emerge from the splintered remains of the old. Despite the tremendous military investment already put forward against them, it is feared that all Tyranid incursions thus far have merely been the vanguard to the truly massive swarm that is even now approaching the galaxy. If all projections prove accurate, then the coming threat may prove to be the deathblow to the already crumbling and over-taxed Imperium and may represent a threat to all other lifeforms in the galaxy.
Minor Xenos Races
Not all sentient species are posed to seize the galaxy for themselves. Hundreds have failed to even prevent their own extinction. Despite this, the galaxy continues to offer a great diversity of alien life in a myriad of forms, all struggling to survive and leave their mark in an increasingly hostile and cruel universe. Below is listed a sample of some of the extinct and still-existing minor xenos species that inhabit the Milky Way Galaxy:
- Demiurg - An intelligent alien race of short, stocky humanoid traders and asteroid miners, they are almost purely space-borne in their massive and majestic commerce vessels. Their history is unconfirmed, as are their origins, intentions and current threat to others. All that is known about them is based at best on speculation. The Demiurg are known to have allied with the Tau, serving as trading partners of the Tau Empire. They were initially introduced to the Tau via their mutual relations with the Kroot. From the Demiurg, the Tau acquired their knowledge of Ion Cannon technology, which is now heavily used by the Tau in their armies and fleets. Two Brotherhoods of Demiurg, the Srry'Tok and Thurm, are known to have joined the Tau Empire outright.
- Diasporex - The Diasporex was a nomadic civilisation of humans and aliens who lived aboard a vast collection of starships, maintaining a network of hydrogen collector space stations around various stars to fuel their vessels' power generators. They had been prowling the depths of space since the Age of Strife and their lifestyle may have evolved as a reaction to the horrors of that time on many human-settled worlds. During the last days of the Great Crusade in the early 31st Millennium, the Iron Hands Legion encountered the fleets of the Diasporex. The Iron Hands offered the human members of the Diasporex the opportunity to separate from their alien allies and to join the Imperium of Man. They declined, and so under standing Imperial orders had to be destroyed as a potential threat to the nascent Imperium. In the months that followed, the Iron Hands Legion fleet tried to destroy the Diasporex, but their fleets proved to be highly skilled and experienced in naval warfare: they easily evaded multiple fleet engagements initiated by the Astartes and even managed to damage an Iron Hands Cruiser. At this point, the Emperor's Children Legion was called in as reinforcements for the Iron Hands. The combined Astartes force eventually made the discovery that the Diasporex used hidden space stations to collect hydrogen fuel from various local stars, which was the reason they had remained in that sector of space rather than just fleeing from the Imperial forces. The Space Marines assaulted these stations and thus forced the Diasporex fleet into open battle to protect them, where they were defeated. A brief shipboarding action occurred, in which the Emperor's Children Captain Solomon Demeter stole the glory of seizing the Diasporex flagship from the increasingly corrupted Primarch Fulgrim. The final words of the alien Diasporex flagship captain were, "We only wished to be left alone."
- Galg - The Galg are a frog-like amphibious alien species who often serve as mercenaries for the Tau Empire. Members of the Galg race are covered in green scales and appear to most humans as large, humanoid frogs. The Tau Empire normally maintains a small number of Galg mercenaries within their ranks but Galgs can be found in the employ of many different factions across the galaxy as long as the price is right. As with most alien species, the Galgs hate Mankind, even when humans serve as their employers, because of the Imperium's intolerance for all intelligent alien races.
- Gykon - The Gykon were an intelligent xenos species mentioned by the Warmaster Horus during the Great Crusade, when he debriefed Captain Garviel Loken of the Luna Wolves' 10th Company. Horus mentioned the Gykon after Loken reported that one of his Space Marines became a Daemonhost during a battle with the last of the insurgents on planet 63-19.
- Khrave - The Khrave are a dangerous species of psychically-gifted xenos that act like slavers and have been plaguing the human race since the earliest days of the Great Crusade. Believed to have risen during the Age of Strife, little is known of the physical appearance of these aliens as all accounts of their human encounters with them emphasise the Khrave’s psychic abilities and their dreadful technology. The Khrave are psychic leeches, feeding on the minds and souls of subjugated human worlds they infiltrate and subdue. The Khrave also seem to possess some kind of natural affinity with the Warp as their preferred weapons are so-called warp-glamours, blades forged from solidified matter of the Immaterium, easily able to cleave through Power Armour. The oldest and strongest Khrave encountered by the Imperium are said to have been able to rival a Primarch in bulk, although these exemplars are thankfully rare.
- Jokaero - The Jokaero are an intelligent alien species who resemble the orange-furred great apes of Old Earth known as orangutans. Though lacking any known form of language, they are capable of tremendous feats of technological construction and manipulation, and specialise in the development of weapons technology, particularly miniaturised weapons. The Jokaero are not a naturally evolved species, but were created by the Old Ones during the War in Heaven over 60 million Terran years ago for the purpose of aiding in their conflict with the C'tan and the Necrons, along with many other intelligent races of the Milky Way Galaxy such as the Aeldari, the Rashan, the Knib and the green-skinned Krork, the possible ancestors of the Orks.
- Kinebrach - The Kinebrach were a species of simian-like humanoid aliens who co-existed with the humans of the Interex culture before the Great Crusade. Long before humanity had spread across the stars from their homeworld of Terra, the Kinebrach ruled a large and mighty interstellar empire. However, by the time the humans of the Interex made first contact with them, the Kinebrach empire was old and decaying. The Kinebrach, rather than go to war against a more vibrant and powerful species, decided to ally themselves with humanity instead. Thus, the Kinebrach were taken under the wing of the Interex, with certain limitations placed upon them. The foremost of these was that no Kinebrach was allowed to carry arms, except in times of war. Those Kinebrach in the Interex military served as shock troops due to their strong, stocky build, which was similar to that of a bipedal gorilla. Kinebrach technology, at the time of the Interex's first contact with the Imperium of Man, was very advanced, in some ways more so than than that of the Imperium at its height - though their military technology lagged somewhat behind - and this was one of the reasons the Interex was able to progress so quickly as a culture.
- Kroot - The Kroot are a species of savage humanoids who are a member species of the Tau Empire and who evolved from avian precursors. A unique feature of the Kroot is that they evolve by selecting traits of their defeated foes to absorb by eating them, unleashing the only known form of Lamarckian selection ever discovered by the Imperium among a xenos species. Kroot who prey extensively on a particular species will begin to take on the anatomical and mental characteristics of that creature. When absorbing the DNA in this manner of sentient species such as Orks and humans, they may also take on cultural aspects of that race as well. In order to gather genetic material from all over the galaxy, the Kroot offer themselves as mercenaries, and sell their services to anyone willing to pay them. They travel the galaxy taking limited contracts from both major and minor races and are regularly employed by the Tau.
- Laer - The Laer were a non-humanoid reptilian species dedicated to the worship of the Chaos God Slaanesh who in appearance were wildly diverse, engineered for each particular theatre of war or cultural function. There were winged, aquatic and all manner of other variations on the basic serpentine Laer form. They were often tall and sinuous, with a snake-like lower body common to their species, and muscular thoraxes sheathed in silver armour, from which sprouted two pairs of limbs. The upper arms bore long, lightning-wreathed power blades, while the lower arms wielded crackling gauntlets that fired lethal green energy bolts. Their heads were insect-like and bulbous, with glossy, multi-faceted eyes and jutting mandibles that produced a grating screech when they attacked. The Laer were first encountered by the Emperor's Children Legion's 28th Expeditionary Fleet during the Great Crusade, concentrated in a single star system on a single homeworld completely covered by a global ocean called Laeran (officially codified in Imperial records as Twenty-Eight-Three). Like the Emperor's Children themselves, the Laer prized perfection in all aspects of life. By the use of chemical and genetic manipulation from birth, individual Laer were best adapted to their cultural roles, whether they be workers, soldiers, diplomats, or even artists. The Primarch Fulgrim refused any notion of co-operation with the aliens, for as Imperial doctrine proclaimed, only humanity was perfect. For an alien race to hold its own ideals to be comparable to those of humanity was blasphemy in its most blatant form, and deserved nothing less than annihilation. In a brutal and bloody campaign the Emperor's Children wiped out the Laeren race. However, the members of the 28th Expedition who investigated the Laer's primary place of worship, a great temple dedicated to the Laer aspect of Slaanesh, were corrupted by the touch of Chaos. Over the next several months and years, Fulgrim and his III Legion -- indeed all those humans who had been exposed to the touch of the Prince of Pleasure on Laeran -- were transformed unknowingly into the sadistic and hedonistic servants of Chaos who joined with Horus in his rebellion against the Emperor.
- Megarachnids - The Megarachnids were a sentient race of non-humanoid xenos resembling giant spiders that resided in the early 31st Millennium on the planet Murder, more formally known as Urisarach and One-Forty-Twenty. An extremely hostile alien species, they were an old enemy of the non-Imperial human Interex civilisation who, instead of committing xenocide, had exiled the surviving members of the species to the world they called Urisarach as an act of mercy. The Megarachnids were stripped of their interstellar travel capabilities, and weather control devices that created powerful atmospheric disturbances and interfered with Vox communications were constructed by the Interex in the shape of large "trees" across the planet to deter vessels from landing on Murder. A campaign of extermination was waged against them by the forces of the Imperium of Man many years later during the Great Crusade after several companies of Space Marines were lost to the Megarachnids' latent hostilities during the initial Imperial investigations of the planet. The war proved costly in Astartes lives, but seemed to be making steady progress by the sixth month. The campaign came to an abrupt end when the Interex arrived to see who was disturbing the Megarachnids' reservation world and first contact occurred between the Interex and the Imperium, as represented by the Warmaster Horus. Whether or not the campaign was continued after the destruction of the Interex by the Imperium in the final days of the Great Crusade is unknown, and the ultimate fate of this species is unrecorded in any existing Imperial records.
- Nicassar - The Nicassar are a non-humanoid, ursine-like xenos species of powerful psykers who are allied with the Tau. The Nicassar are driven by an insatiable curiosity to explore and travel across the galaxy. It was this drive which led a flotilla of Nicassar Dhows to make contact with the Tau, resulting in the Nicassar becoming the first non-Tau race to become an addition to the fledgling Tau Empire. Their contribution to the Greater Good is the provision of starships for the Tau Navy, particularly vessels involved with reconnaissance and exploration duties, which are often crewed by the Nicassar themselves rather than the Tau Air Caste. The propulsion and navigation of all Nicassar vessels stems from the potent psychic powers that define the Nicassar species. For this reason, the Tau have been careful to prevent the Imperium of Man, which is known for its extreme persecution of psykers, from discovering the existence of the Nicassar within their empire. The Nicassar are utilised by the Tau only for spacefaring; their own limited mobility makes them highly ill-suited for ground combat.
- Rak'Gol - The Rak'Gol are vicious xenos marauders and a relatively new threat within the Koronus Expanse of the Halo Stars adjacent to the Calixis Sector. The species was encountered by the Imperium first in the dim stars past the Alenic Depths in the 8th century of the 41st Millennium. A xenos breed of which little is known for fact, they take the appearance of rough-hewn and irregular stone reptilids, eight-limbed and over three metres long. Chalky white in colour and mantis-like in bodily arrangement, Rak'Gol warriors favour cybernetic augmentation to increase their abilities and replace lost limbs. Their point of origin remains unknown, as does the motivation for their sporadic attacks on human-held worlds and vessels, other than to slaughter indiscriminately and steal minerals and weapons. Their rasping, screeching language remains incomprehensible, and no successful communication between the Rak'Gol and humanity has been recorded; no Rak'Gol has ever been captured alive. Indeed, even their name is taken from children's stories of mythical monsters from the settlement of Footfall's slums, legends which they superficially resemble.
- Tarellians - The Tarrelians are a minor reptiloid species. Anatomically they are narrow-waisted and possess broad shoulders, while they stand slightly shorter on average than humans. Because of their long snouts and their predilection for working as mercenaries, they are often referred to as Tarellian "Dog-soldiers" by humans. During the Great Crusade, Imperial Expeditionary Fleets virus-bombed most of the Tarellian homeworlds, almost wiping out the entire species. This has resulted in the Tarellians bearing a grudge and eternal enmity towards the Imperium that has lasted for ten millennia. In 998.M41, a Tarellian civilisation was devoured by the Tyranid Hive Fleet Moloch during its advance from the galactic north and the Kiltor Sector.
- Tushepta - The Tushepta are an alien race spoken of during the Great Crusade by the Warmaster Horus during his debriefing of Loken, the Captain of the Luna Wolves' 10th Company, after one of his Space Marines became a Daemonhost during a battle with the last of the insurgents on planet 63-19.
- Vespid - The Vespid are a vaguely insectoid species, though this narrow term is an imperfect one that fails to account for many aspects of their unusual physiology. Their bodies are encased in a chitinous exoskeleton and sport many lethally sharp barbs. They see by way of three pairs of eyes, one pair perceiving the ultraviolet range, one the normal visible spectrum, and the last the infrared. It is assumed that the Vespids see in all three ranges of the electromagnetic spectrum simultaneously, and therefore have a perception of their surroundings unique to their race. Vespids can fly by way of a pair of hard chitin wings, which also emit a continuous ultrasonic tone with which the species appears able to modulate and control the workings of their crystal-based technology. The Vespids are a member species of the Tau Empire.
In the long millennia since Mankind first began to settle amongst the stars, some human colonists have deviated to a great degree from the base-line human genome due to exposure to the alien environments of their homeworlds and long centuries of isolation from the rest of humanity during the Age of Strife. These sub-species of humanity often differ in physical and cultural terms from the human baseline to such an extent that they are almost as inhuman as true xenos. These variant human sub-species are generally referred to by the Administratum as Abhumans.
- Interex - The Interex was a highly advanced interstellar human civilisation that existed across some thirty star systems at the time of the Great Crusade in the early 31st Millennium. The Interex, unlike the Imperium of Man, had decided that the best way to deal with intelligent alien species was not to exterminate them or make war upon them, but to try and coexist with them. In fact, the Interex had become the close allies and nominal overlords of a simian alien species called the Kinebrach who they had integrated into their own society. The Interex ultimately became enemies of the Imperium after hostilities between the two human civilisations began when the Interex accused a diplomatic party of Luna Wolves led by the Warmaster Horus of breaking into their Hall of Devices, a museum on the Interex world of Xenobia dedicated to weaponry, and stealing a prized Kinebrach blade called an Anathame that was known to have links to Chaos. The Interex believed that the Imperium was allied to Chaos (which they called "Kaos") and never knew that the weapon had in fact been stolen by First Chaplain Erebus of the Word Bearers Legion, who had been with Horus' diplomatic party. Unknown to Horus, Erebus and the Word Bearers Legion had already turned to the worship of the Ruinous Powers of Chaos. The powers of the Warp had informed Erebus and his Primarch Lorgar that if they wished to bring Horus over to the side of Chaos, then they would need the Kinebrach Anathame. The Interex civilisation was ultimately wiped out by the Luna Wolves, following the commencement of hostilities between the two human factions on the night Erebus stole the Anathame.
- Ogryn (Homo sapiens gigantus) are a huge and physically powerful Abhuman mutant subspecies of humans, often employed as shock troopers in the Imperial Guard by the Imperium of Man. Ogryns possess many traits prized by the Imperium; they are brutally strong and completely loyal, although extremely limited intellectually, which limits their battlefield role to simple and direct assaults. They come from cold and barren planets in the galaxy with high gravity, such as Anark Zeta, which is why they appear larger, heavier and bulkier compared to baseline humans. An Ogryn's speech is much the same as an Ork's. The most famous Ogryn in Imperial service is Nork Deddog.
- Olamic Quietude - The Olamic Quietude was a technologically advanced cybernetic human civilisation that had been founded during the Dark Age of Technology and over the course of 15,000 standard years merged themselves so completely with their advanced technology -- as a possible result of the influence of unknown xenos -- that there was little left of them that could be considered truly human. The Quietude lived in vast and deep techno-hive cities protected by an artificially-extended icecap on their unnamed homeworld, an otherwise lifeless but heavily defended planet. They were encountered by the Imperium of Man's 40th Expeditionary Fleet near the end of the Great Crusade in the late 30th Millennium, and utterly annihilated by the 3rd Great Company of the Space Wolves Space Marine Legion, another of the Abhuman pretenders to the human future wiped out by the forces of the Emperor of Mankind.
- Ratlings (Homo sapiens minimus) are small, loud, hungry and lecherous Abhumans. Ratlings are granted full Imperial citizenship despite their mutant status and in the past have often served in the Regiments of the Imperial Guard. However, they are still distrusted by the more Puritanical members of the Inquisition.
- Squats (Homo sapiens rotundus) - The Squats are a short, stocky and physically hardy Abhuman sub-species of Mankind who were adapted to the heavy gravity conditions that predominated on the worlds they had settled near the core of the Milky Way Galaxy. Of all the Abhuman types encountered by the Imperium, they most closely resembled baseline humans. Squats were the descendants of baseline humans who had colonised the worlds around the galactic core in the far distant past of the Dark Age of Technology. These worlds are some of the oldest in the galaxy, formed when the galaxy's structure had not yet been fully stabilised. The Squat species was ultimately destroyed by the invasion of a Tyranid splinter Hive Fleet that consumed their homeworlds in the late 41st Millennium, though some survivors still see service with the Imperial Guard, hoping for revenge and to find a new beginning for the remnants of their race.
Xenos is a word that originated from the Greek language. The most common definition is "stranger." However, it can be interpreted in different ways, depending upon who is using it or what situation it is being used in, producing variant meanings such as "enemy" or "foreigner."
- Codex: Craftworld Eldar (3rd Edition)
- Codex: Dark Eldar (5th Edition)
- Codex: Dark Eldar (3rd Edition)
- Codex: Eldar (4th Edition)
- Codex: Eldar (3rd Edition)
- Codex: Eldar (2nd Edition)
- Codex Imperialis, by Rick Priestley
- Codex: Necrons (5th Edition)
- Codex: Necrons (3rd Edition)
- Codex: Orks (4th Edition)
- Codex: Orks (3rd Edition)
- Codex: Orks (2nd Edition)
- Codex: Tau Empire (4th Edition)
- Codex: Tau (3rd Edition)
- Codex: Tyranids (5th Edition)
- Codex: Tyranids (3rd Edition)
- Dark Heresy: Core Rulebook (RPG)
- Dark Heresy: Creatures Anathema (RPG)
- Dark Heresy: Disciples of the Dark Gods (RPG), pg. 71
- Dark Heresy Timeline
- Deathwatch: The Achilus Assault (RPG)
- Deathwatch: Core Rulebook (RPG), pp. 358-374
- Deathwatch: The Jericho Reach (RPG), pp. 56-204
- Deathwatch: The Nemesis Incident (RPG)
- Deathwatch: Mark of the Xenos (RPG)
- Deathwatch: Oblivion's Edge (RPG)
- Digganob (3rd Edition)
- Ere We Go!: Orks in Warhammer 40,000 (1st Edition)
- Gorkamorka: Da Uvver Book (2nd Edition)
- Planetstrike (5th Edition)
- Rogue Trader: Core Rulebook (RPG), pg. 359
- Rogue Trader: Battlefleet Koronus (RPG), pp. 73-101
- Rogue Trader: Fallen Suns (RPG), pp. 43-55, 62-67
- Rogue Trader: Edge of the Abyss (RPG), pp. 70-74
- Rogue Trader: Into the Storm (RPG)
- Rogue Trader: The Navis Primer (RPG), pp. 133-135
- Rogue Trader: The Koronus Bestiary (RPG), pp. 85-91
- Rogue Trader: Stars of Inequity (RPG), pg. 48-49, 82
- The Art of Warhammer 40,000
- Waaargh!: The Orks (1st Edition)
- Warhammer 40,000: Apocalypse
- Warhammer 40,000 Compilation
- Warhammer 40,000: Compendium
- Warhammer 40,000 Rulebook (6th Edition), pp. 196-213
- Warhammer 40,000 Rulebook (5th Edition)
- Warhammer 40,000 Rogue Trader (1st Edition)
- White Dwarf 241 (US), "The Torturer's Tale: Dark Eldar Background"
- White Dwarf 228 (US), "Codex Dark Eldar"
- White Dwarf 173 (US), "Eldar Pheonix Lords", pp. 6-9
- White Dwarf 172 (US), "The Eldar", pp. 5-16
- White Dwarf 124 (US), "Eldar: Jes Goodwin's Sketch Book," pp. 30-33
- White Dwarf 105 (US), "Harlequins" by Graeme Davis, pp. 35-46
- Xenology by Simon Spurrier
- Courage and Honour (Novel) by Graham McNeill
- Eisenhorn: Xenos (Novel) by Dan Abnett
- The Flight of the Eisenstein (Novel) by James Swallow
- Fulgrim (Novel) by Graham McNeill
- Horus Rising (Novel) by Dan Abnett, pg. 199
- Kill Team (Novel) by Gav Thorpe
- The Last Ditch (Novel) by Sandy Mitchell
- Midnight on the Street of Knives (Novella) by Andy Chambers
- Path of the Renegade (Novel) by Andy Chambers
- Soul Hunter (Novel) by Aaron Dembski-Bowden
- Wolfblade (Novel) by William King pg. 71
- The Horus Heresy, Book Three : Extermination by Alan Bligh, pp. 151-152
- Imperial Armour Volume Twelve - The Fall of Orpheus, pg. 21