"xenology - n. 1. The science of foreign or exotic things. 2. The science of parasites. [from Greek xenos stranger, logos science]"
It's a science-fiction trope to have a scientist whose specialty is "xenology" (or "xenobiology," "xenochemistry," etc) whose job is to be the expert at anything that would not be encountered on Earth, and thus exposition for the sake of the reader of the story. In real life, this is known by names such as astrobiology and astrochemistry, while xenobiology consists of trying to design lifeforms with biochemistry or genetic makeup not found on Earth.
Sci-fi RPGs will probably have a 'xenology' skill, used for tests involving aliens or off-world artifacts.
"Xenology" is the title of a Black Library sourcebook, which purports to be Inquisitor Brehm Sasham's account of his investigation of a rogue Magos Biologis working in the employment of Radical Inquisitor Lord Maturin Ralei (who was in fact a Necron Lord in disguise). Along the way, he publishes the Tech-priest's notes on dissected aliens and xenotechnology.
Many interesting details of alien biology are given, some of which are a little dubious, like toes on a Tau. Other details are just weird, like Eldar having crystalline feces (that ought to hurt) and extremely sensitive ears.
It's unknown why these discrepancies are present, although the Tech-priest in question does go increasingly insane as the book progresses until he completely snaps and "dissects" the Inquisitor "alive" at the behest of said Necron Lord. Additionally, the Necron fluff mentioned is obviously pre-Newcron in origin which opens up a few other contradictions as well.
While the book itself is an impressive collection of dissections that would be of interest to any 40k fan, what's actually more significant is the overall plot of the book itself. Several successive revelations are made as the study of each species continues, leading to one of the broadest and most in-depth analysis of the Warhammer 40,000 universe on the whole, including information on some xenos species that are mentioned in no other source.
While the main theory is that the Necron Lord's master may be The Deceiver the new fluff development in recent editions may point out that the whole book plot was just another of Trazyn's ploys in order to increase his personal collection with some new fine biologist curiosities. Or he is impersonating a C'Tan shard too.
Since it's been out of print since 2006 and based on older fluff that's no longer true (such as Orks having green/black blood; as of 6th edition onward they have red blood) many fa/tg/uys treat it as no-longer-canon in its entirety. As such, whether or not it's canon -- especially its decidedly non-rodent depiction of the Hrud -- is a discussion most assured to derail any 40k thread that steps on this little continuity landmine.
While not the same in goals, there's also 2021's Liber Xenologis, a more modern book written in the perspective of Rogue Trader Janus Draik of Blackstone Fortress fame. It also offers insights into the various xenos in the galaxy, but the viewpoint of someone who has either parleyed with or hunted down the various xenos he's dealt with. That said, it doesn't go into the more specific biological processes in a medical fashion like Xenology.
Dissection 1 - Thyrrus
The first specimen to be "studied" is the aforementioned Thyrrus Warrior which basically appears as a cross between a Chaos Spawn and a squid, with the ability to change colour at will and take in nutrients from the atmosphere by selective osmosis (this actually causes a problem where the Magos cannot gas it to death as expected).
- Analysis of the xenos society shows a stupid preference for spectacle warfare, where they go to war merely to show off, rather than follow any predictable pattern. They'll throw heavy tanks at lightly defended fortifications, and then send wave after wave of infantry at unassailable defenses. They have no understanding of "victory" or "defeat" and have no concept of free will, rather they believe that everything is just one big pantomime and they all do their best to show off with pretty colours and big explosions - and so, Games Workshop managed to make a reference to neckbeards anywhere in universe.
Dissection 2 - Tau (Ethereal Caste)
The information gained from interrogating the Tau isn't really new to a 40k player, though the Greater Good was news to the Magos. As for the dissection: Tau have generally poorer eyesight than humans but a better sense of smell. Internally; they have one lung and multiple stomachs and their immune systems are centralised (unlike humans) with the heart doubling as a liver and kidney, while their skin doubles as lymph nodes.
- One VERY interesting factoid is that ethereals have a crystalline organ in their skull cavity of unknown purpose (yet... read Q'Orl) but is somehow connected to their manufacture of pheromones. It's hinted this organ may have been engineered, but by who is unknown. Weirdly enough, the dissection shows that the Tau have toes despite all other artwork portraying them with hoofs.
- This is not wrong as it is seen in models were the hooves are visible that they have cloven hooves as well as a dewclaw. A hoof is just a very large toenail that wraps around the end of the toe that the animal or Xenos walks on. So yes, Tau do have toes.
Dissection 3 - Eldar
Eldar have crystalline shit...yes, really. The study itself shows that Eldar biology is extremely efficient with little to no waste products or redundant organs, they have twenty DNA nucleotide bases instead of our four and have a quintuple helix (this may be a retcon, as other sources have stated triple helix). Not only that but they can apparently change their DNA subconsciously (explaining why the Craftworld path system can apply to everyone, and how psykers can "retire" their powers and become "safe" again although all Eldar remain psykers even then). The Inquisitor suggests that the common features shared between many of the galaxy's races (including humans, which is extra heresy) suggests a common ancestor or galaxy-wide genetic engineering. The most important note to all of the /d/eviants on /tg/ is that it specifically mentions eldar ears as being an erogenous zone, containing many thousands of nerve clusters.
- Psychic study of Eldar artefacts also pretty much confirms the four subraces are distinct (psychically at least), with Craftworld Eldar feeling lost and resigned, Dark Eldar being in a mad panic to cause pain & destruction to avert the gaze of Slaanesh, Exodites believe they are going to rebuild the species (but no their empire) and Harlequins are laughing for the sake of it.
Dissection 4 - Umbra
An Umbra is a black ball 8 centimeters in diameter. These little fuckers are pests found attached to warp engines and near entrances to the webway. The dissection proves nothing, except they are damaged by light and contain no organs except for an unidentifiable non-Newtonian liquid (ie: it changes viscosity based on applied force). When the specimen was killed, it left an image of a humanoid figure being broken into pieces, followed by images of a warp storm and deep space, as well as a psychic shriek of the word: "LINGER" in the observers' minds.
- Anecdotal study shows these pests are FUCKING DANGEROUS, they have the ability to manipulate and solidify darkness into shapes and weapons, Even manipulating the dark spots inside the pupils of its victim's eyes to create hooks, barbs and teeth. There is also a picture of these little spheres assaulting the Ultramarines.
- Bits of evidence collected by the Inquisitor in the anecdotes surrounding the Umbra indicate that around the time of the War in Heaven, one of the Old Gods disappeared to hide in the warp and watch the universe from there. Then later, Slaanesh was born and LOL'd at him, chopped him into a million pieces and kicked him back into the material universe to linger forever.
- The Inquisitor suggests that the Umbra are a three dimensional representation of a multidimensional being, which is why they appear so simple, when in fact we cannot bend our minds in such a way to actually see it for what it really is.
- The Inquisitor also speculated that the motive for the Umbras hostility towards the material races might be rooted in this ancient story, whether out of confusion or anger, they are trying to remove the mortal races psychic influence on the warp and allow it to become calm once again.
Dissection 5 - Ork
When not in constant warfare, Orks get fat; the ork that was locked up in the Inquisitor's cells was obese. DNA showed that Orks DO indeed have fungal DNA and it contains all the genetic information of the entire species and its subraces, again suggesting that they have been engineered in some way. They can accept any number of prosthetics and have absolutely no problem accepting donor implants.
- As is well known, some Boyz are born with an instinctive understanding of things derived from their DNA. The Inquisitor goes a step further and suggests that when Ghazghkull Mag Uruk Thraka was first wounded in the head by a bolt shell, he might have unexpectedly tapped into his DNA and unlocked an entirely new caste of Ork never seen before. (he does claim to have visions from Gork & Mork - possible genetic memory?)
- Weedkiller works just fine on Orks, go down to your nearest garden store and buy some today. This was proven when the Magos cultured some ork cells taken from the corpse, found them growing and spreading exponentially, freaked out and purged them in fire and chemicals.
Dissection 6 - Kroot
Kroot need to eat to evolve, their reproductive system is built-in to their digestive system. You eat something, you pass it on, so their homeworld of Pech contains many apex predators constantly evolving as they consume one another. The creative portions of their brains are literally smaller and less-developed than the logical-rational ones -- think a human with a bigger left half of the brain than the right (yes, we know it's debatable that this is how the human brain actually works, but Kroot aren't human).
- It is strongly suggested that Kroot therefore evolved into what we know today entirely by accident and that Orks crashed onto their planet early in their history. Not only causing them to become vaguely humanoid and intelligent, but Kroot CAN tap into the information contained in Ork DNA, and could therefore intuitively build devices even though they had no idea how to do it and would have difficulty developing such tech on their own. However, that was ancient history and modern kroot are presumably much more evolved and diverse since Shapers took over and directed the tribes eating habits to prevent them from moving into genetic dead-ends.
- It is also hinted that the Tau are hindering the evolution of the Kroot by forcing them to curtail the worst of their eating habits, and thus the intake of useful new genetic material.
Dissection 7 - Q'Orl
The Q'Orl is a species of giant space ants with a relatively large empire in the (pointedly) galactic west. They have a caste system of workers and queens, and are an advanced race with technology and starships, but they have no psykers so the only way they can travel long distances is to capture and implant Navigators and Psykers drawn from other species.
- The dissection shows that they ALSO have a crystalline organ, the same as the Tau, built into their pheromone systems, which then leads the Magos to start ranting about proof of genetic tampering between the races, since the Tau and the Q'Orl could not be further apart galactically.
- One story indicates that in late M37, the Eldar stole a queen from the Q'Orl with the intention of creating an "uncorroded swarm".
- It is therefore tempting to think that the Eldar had a hand in the evolution of the Tau Ethereals. The timeline matches roughly according to the 4E Codex, but the Eldar have never made any suggestion of crafting the Tau race nor have ever been hinted at directing galactic evolution in the past.
- The dates and identification of the aliens involved as Eldar is somewhat dubious in-universe, since it involves quite a bit of speculation on the meanings of mythic descriptions. All it really says about the race involved is that they are looking superficially like humans to an alien insect, except taller and thinner. It is a common description of Eldar, but there are plenty of races in the Galaxy, not to mention holographic tech.
- That the Eldar assisted the Tau in finding and reverse-engineering Imperial technology from "crashed" colony ships ("helping" as in "doing it for them because you can't reverse-engineer something more advanced than you), and later abandoning the Tau for their "crippling lack of creativity" suggest that the Eldar were involved with the Tau. It does not mean that the Eldar were the ones who created the Ethereals, however, and they might have simply seen a species that looked sane and had the means to unify minor aliens. Perhaps the Cabal isn't so dead after all.
Dissection 8 - Various Tyranids
Of note, the digestive tract of all the Tyranids is very primitive, in stark contrast with the rest of their biology. The Genestealer and Ripper posses grinding plates above their stomach-analogue which reduce any food to liquid, enzymatic action processing it into a protein soup which is then pumped around the body. The Lictor and Warrior did not possess the grinding plates, and it's speculated they only consume a liquid nectar-analogue supplied by the swarm.
Oh, Rippers also have no means of excretion, and comes with pre-fertilized eggs. They're born, they eat, they reproduce, they die, to be consumed by something more important.
Dissection 9 - Hrud
The Hrud is a freakish trash-monster. Its death caused its whole body to liquify leaving not much left to work with. However the creature had no "skeleton" per se; instead it was made up of a series of vertebrae, meaning it can bend itself in any direction. They also have hard silicate pauldrons which seem to double as waste excretion organs, meaning their bodies can divert toxic, bacterial and viral by-products into their upper body as a defense mechanism.
- Evidence shows that they live amongst humans, in the depths of hives or the bowels of starships, entire clans! This is made possible because it is hard to look at Hrud, they appear perpetually shrouded and difficult to focus on, though it is unknown if it is a biological or psychic property. It's also impossible because psykers aboard the ships, especially astropaths, would have quickly noticed the Warp shenanigans going on.
- It is also known from other sources that they can manipulate the flow of time, causing creatures and structures to age and weaken rapidly. (An entropic field, specifically, whether by accelerating time or specifically accelerating the concept of degradation is a matter of skub)
- Their society is explained in one source where a human pet-slave was recaptured and interrogated by the Imperium. This slave reveals that the Hrud deities were real and physical entities (to them, but as they have an unparalleled racial memory and more efficient record keeping who's to disagree?) and that their pantheon consisted of the same archeotypes of a red-handed god, a hunter, a laughing god and an artisan god and that around half a million year ago, one of their deities; Qah (he who LINGERS) disappeared from their racial memory but promised to return. (but perhaps did not count on being chopped up by Slaanesh). Given what happened during the Umbra dissection, it is strongly inferred that Qah and the Umbra are related somehow.
- It is also hinted that the Hrud racial memory perhaps extends further than the present-time, since from their perspective, the human Imperium is a thing that no longer exists, and fell millennia ago.(or their ability to manipulate the flow of time is related to this) (or a reference to the Horus Heresy effectively "ending" the Imperium as it was meant to be and the current Imperium is pretty much something else entirely)
- At the First layer, there are representations of the Gods creating the creatures of the second layer
- The Second layer shows species with recognisable features taken to represent Eldar, Orks and others.
- The Third layer shows a featureless, skull faced and mysterious enemy in opposition to the gods of the first layer, and that the species of the second layer will be significant to the outcome of the conflict.
- The Fourth layer represents those gods of the first layer that were the survivors of the war. A laughing/crying figure is one survivor, and a bloody handed figure was the second (though the depiction was vandalised by the Eldar themselves, no doubt after his ass-whooping by Slaanesh). A third survivor was missing from the tablet, but was recreated using other sources, but shows a unborn child enveloped in sunlight, which considering it's Eldar origins would represent Ynnead which is yet to be born from the webway.
- This missing representation could be an indication that the Star Child and the Eldar God Ynnead are one and the same thing and that the Emperor and his webway are all part of the grand design...perhaps the Emperor during the Great Crusade was trying to wipe out the Eldar to accelerate Ynnead's birth.
- A damaged and mostly missing fifth layer of the tablet only shows either wings or flames, depending upon your opinion or angle of observation.