The remnants of the long range trade and transport fleet of the Eldar Empire have recognized the extent of cruel decadance to which the main Empire have fallen. They mostly have recognized the danger early and fled the destruction, taking with them whoever agreed to come. Later, being on the run from pretty much everyone and unable to reasonably establish the complex infrastructure their standards for colonization demanded, they repurposed their giant ships into permanent mobile habitats.

Now, to protect themselves from Slaanesh's claim, they walk Paths of abstinance and devotion to their craft, and wear devices called Soul Stones that preserve Eldar souls within them upon death. Those souls are later offloaded into storage banks inside their ships called Infinity Circuits.

Their society is essentially technocratic, with the main decisionmakers being those Eldar who choose - and have the mental and psychic ability to progress the furthest on - the Path of the Seer, named Farseers. Farseers devote their lives to prediction and shaping of fate through both psychic influence and mundane strategy, and the predictive power of a sizable council of Seers working together is pretty much only surpassed by Tzeentch himself, their plans successfully spinning millenia.

Almost none of them know the rule of succession. Not as a formula, anyway.

Infamously, the lives of other races don't weigh much at all in their decisionmaking processes. Millions of souls of other species, and some species in their entirety, are routinely sacrificed in complicated schemes to save dozens of Eldar. 

Now, granted, most of those other species would kill millions of Eldar for no reason at all other than them being there. But they would do it in an honest and straightforward way of a brutal war campaign, rather than by dirty scheming.

WHICH IS EVIDENTLY MORE SYMPATHETIC ACCRODING TO AT LEAST SOME PEOPLE.

THAT IS TO SAY.

FUCK THE EEEEEEEEEELLDAAAAAAAAAARRRRRRRRRRRR!!!!!!!!!!!!!

Already on it, sweetie!

...

...anyway.

Craftworld Eldar are a major faction, but they are dying out. Their Craftworlds are being picked off one by one and they can't build more.

They possess incredibly deep knowledge of psychic technology and, actually, better than average knowledge of hard technology. But they badly, *badly* lack the manpower and resources to use it. And attempts of transmitting it are met with obstacles of the Eldars' own pride and secretiveness, the others' xenophobia and mistrust, and surprisingly deep communication barriers.

(They are a few radical-faction Imperial Inquisitors and a couple Tau research outposts that are on the case, of course. But it probably just isn't enough.) 

For all of their craftiness and magic expertise, the Craftworld Eldar are struggling to survive. Perhaps the biggest obstacle in their way is their own clinging to their traditions and ways of life. Deep down, the wisest among their Seers are aware that treating everything as an art is crippling, that repeatedly manipulating everyone and treating all non-Eldars as barely sentient is crippling, and that they would do much better as a loyal force-multiplying ally to a different faction than as lone wolf survivors.

But, for most of them, this sounds too similiar to Eldar extinction.

And then there's the Dark Eldar.

If you thought that the forces of Chaos were sick and twisted, you thought wrong.

Before the collapse of the Eldar Empire, there were three groups of Eldar. The first group felt sick about the depravity and decadence of the Empire, and it got away and later formed the modern Exodites, Craftworlders and Harlequins. The second, and by far the biggest, group felt right at home, and they got devoured. The third group felt that the problem with the Eldar Empire was that there wasn't enough decadence and depravity.

So they retreated into shady corners of the Webway, beyond the reach of police and various other killjoys, to pursue even more interesting forms of pleasure.

And when the Fall happened, they weren't hit with the first strike of the psychic scream. But they were indeed branded with Slaanesh's claim, which, given their hedonistic lifestyles, began to chew on them quite quickly.

Except they have found a solution for it.

By witnessing and enjoying overwhelming amounts of pain of those nearby, the curse of Slaanesh could be staved off. This solution was, of course, discovered really quickly by these denizens of the Webway.

Some say that that satisfies the tug upon their souls that Slaanesh projects, feeding her the suffering and souls of the innocents instead of their feelings and souls.

But that doesn't, actually, make sense, now does it, beauties?

Because if it was so, why wouldn't it work on witnessing the pleasure of others rather than their pain, given that I feed largely on pleasure?

If it was so, why would torturing a few subjects in an arena, as is the common way for the poorer Dark Eldar to feed, be enough to satisfy an entire crowd?

If it was so, why wouldn't I give them, constant suppliers of power for me, a pass, or, indeed, broker a pact with at least some of them, instead of hunting them down as I do?

Because the way their solution works is by creating and bathing in so much suffering, it's warp trace is anathema to me. Yes, I am a God(dess) of sadism, but I always mix pain with pleasure. Because over and above being a deity of sadism, I am a deity of pleasure.

Whatever the hell these people are doing, it is completely antithetical to pleasure, and indeed most emotions and mindsets ever generated by anything, no matter how dark their existence. And that is why even daemons are repulsed by Dark Eldar.

I don't feed on that. Khorne, Nurlge and Tzeentch don't either. But Warp remains Warp. So someone else does.

Some have suggested that the Dark Eldar would be strong bidders on the intergalactic masochist market. But, actually, masocists are inconvenient for their purposes. Pain that is enjoyed doesn't count.

Dark Eldar hunt for slaves, breed slaves, and, of course, take each other for slaves.

The souls of their slaves, all of whom eventually die of unbelievable extreme pain (if nothing else), are shattered, and some of their pieces are indeed absorbed into the essence of the nearby Dark Eldar, elongating or even reversing their aging. This worked before Slaanesh, but wasn't noticed or researched, because those those Eldar who tortured others into states that extreme weren't really into statistics.

The shady brothels, drug dens, pubs, dungeons (of both kinds), black markets, fighting rings and illegal laboratoroies of the Webway swelled up after the Fall, as everyone who could have escaped there to find solace from the onslaught of demons and soul-rending screams. Over time, they got interconnected and grew into each other, and the great Webway-city of Commorragh was born. Now, it's surface area is equivalent to a surface are of all the planets in an entire medium-sized solar system.

The city of Commorragh has two rulers.

The first is Moloch. 

The second is Asdrubael Vect.

Remember how, for the Eldar, everything is art and everything is mysic?

For the Dark Eldar, the biggest arts are torture and betrayal. Their mysticism finds an outlet in worshipping and imitating their biggest and vilest historical criminals, called the Dark Muses, and I am not making this up. Inside the Commorragh, there really is no law except might making right and the fittest surviving. Just how well this latter part works can be seen by how they, a race created to be psykers, have already bred most of their psyker potential out in their strains, as it makes their method of shielding from Slaanesh way harder - nigh-impossible if used actively. There's no currency (beyond the natural but inconvenient currency of slaves), no property record, no social services or police or army or anything of the sort. Dark Eldar cluster into Kabals which put out their own armed forces to defend against the invaders and each other. All leadership positions within them are always up for grabs, of course, and turning your back on your fellow Cabalites for two seconds will probably result in you being enslaved and tortired for sustenance and shadenfreude. There is a single universally respected tradition, however, to abstain from infighting during the slave-taling raids. Kabals reliant on such raids that fail to universally uphold it, of course, suffer a swift and miserable destruction.

On the flipside, Commorragh is, without a hint of irony, the most welcoming and open community in the Galaxy. Aside from Tyranids, or those affiliated with Chaos, or those too convenient for Chaos (i.e. psykers who can't hold it in their pants) - who would be immediately simultaneously murdered by everyone nearby out of fear for their souls, everyone is welcome, and potentially respected, here. The Dark Eldar have long realized that Eldar and other races are very much the same when you chain them to the wall and torture them to insanity, and the sheer open Darwinism of their lives have taught them not to underestimate their slaves or competitiors from othet species. Many races, including Humans, Orks, and even Tau can be found listing as members of the Kabals (which really tells you something about the spiritual unity of all peoples of the Galaxy). Very occasionally, slaves who demonstrate truly extraordinary talents at torturing others are freed and taken by Dark Eldar torturers as apprentices. Fundamentally, the difference between s a slave and a slaveowner is purely a matter of one's skill.

The unhinged nature of life in Commorragh coupled with the novel and unprecedented need to maximize torture have not only cleansed bigotry from the minds of dark eldar but also spurred their creativity and curiosity, and increased their tolerance for formal analysis. Thus, Commorragh is also one of the most technologically advanced societies in the Galaxy. For obvious reasons, they focus on biological manipulation, but some also possess limited nanotechnology and some can do things like putting black holes into boxes. Of course, there are no networks for distribution of that knowledge, so it spreads mostly through industrial espionage and underhanded dealings - so, quite slowly.

There are other advantages to such an aggressively selective environment. The speed of reproduction have went way up, and given that Dark Eldar society isn't really constrained by any resources within it territory, Commorragh is in perpetual population boom. In addition, the environment is so brutal that most invasion attempts simply feel like brief and mild intensifications of same-old same-old, if that. Lastly, the Dark Eldar with developed compassion, or apathetic deppression, or an overabundance of fear due to a constant risk of death, or any other psychological trait that would make living here miserable, were simply bred out.

Aside from not having any grand ambitions or long-term plans, they fully deserve a title of a major faction.

And, out of all the peoples in this Galaxy, Dark Eldar are a rare one that does, actually, on average, have a lot of fun.

...stingy bastards.

You know who else has a lot of fun?

So, The Orks were concieved as a biological weapon in a long-finished war. They were made to resist the scorched earth methods of the enemy by quickly and automatically terraforming planets completely burnt to ash into warrior-producing factories.

That war have long since ended, but the Orks had no off-switch installed. Or maybe they did, but the Old Ones never activated it, because, uhh, ummmm... 

Mayhaps.

Well, the Orks have since degraded into small warrior-tribes. But they are still made for fightin' and winnin'. They still grow into combat-ready colonies from a single spore. They still have knowledge of technology in their blood. They still generate plenty of psyker energy, which is funneled into a shared field called WWWAAAAAAAAAAAAAGHHHHH!!!!!!!!!, that direcrly alters reality according to their perceptions and in their favor, without all the annoying and dangerous Warp interfacing. They still have enough muscular power to easily pummel anyone except perhaps Tyranids in close combat.

And they are always down for a fight.

Usually they fight other Orks. Because other Orks are the ones most often within punching distance.

Ork culture is centered on relishing in combat and destruction. And the most important fact about it is that Orks don't actually know what the ass they are doing, at any given time. But their psychic field, as well as their sheer audacity, often make it work anyway.

Orks use their teeth as currency, with the default method of obtaining it being punching other Orks. They see no obvious problems with this method.

Orks use height as their default method of selecting a leader. They see no obvious problems with this method.

Orks jump around the Warp without navigation or shielding from the demons, moving along a general direction as opposed to a conrete destination. They see no obvious problems with this method.

Orks believe that red vehicles go faster. Which, around the Orks, they do. They also believe that purple is the sneakiest color.

And, really, when is the last time you've seen a purple Ork?

Orks have an advantage of numbers. Though Imperium is big as a (sort of) cohesive unit, as of right now, if you ignore disunity and just compare populations, the Orks are by far the biggest faction in the Galaxy!

You know. So far.

But the Orks also have an additional secret advantage:

Orkses is never defeated in battle. If we win we win, if we die we die fighting so it don't count. If we runs for it we don't loose neither, cos we can come back for annuver go, see!

They are the Orks. They always win. Especially when they win.

And now for a different collection of merry bands of slaughtering conquerors. The Forces of Chaos!

Objectively speaking, Chaos is the faction with the furtherest reach. Dark Eldar are largely limited to a small part of one of the layers of the Warp, everyone else is largely limited to a continuous patch of realspace. Chaos controls an entire layer of Warp and maintains military supremacy over an another one; it has a huge permanent outpost in realspace in the Eye of Terror, it controls tens of thousands of planets of various species randomly scattered across the Galaxy, and maintains cults and infiltrators and contacts in many more. Every Psyker without sufficent discipline risks becoming it's agent, and every religion risks being subtly subverted by it. Through dreams, it influences mankind.

It's resources are vast. The Four, beings of ancient and astronomical in it's scale power, get stronger from from every living person with at least some psychic sensetivity who experiences rather typical emotions of frustration, resignation, hope, and joy. Much more of such power is generated by intentional worshippers. Daemons of Chaos, though not nearly as numerous as Orks or Tyranids, possess much more magical power per capita, can naturally use FTL travel, are tireless, have no biological needs, eventually respawn upon death unless their God wills otherwise, and, with their mere presence, spread the chaotic Taint that is usually really hard to get rid of for good. Of the army that conquered the Galaxy for mankind, Chaos now commands an entire half, including half of the legions of Space Marines, traitor fleets of the Navy, traitor batallions of Imperial Army and traitor subdivisions of Adeptus Mechanicus. The forces of Chaos restructure the very fabric of reality on Daemon Worlds to allow them to cheat at industry, and have free access to things others would fear to touch with a ten foot pole. Chaos Gods command interstellar epidemics, stir rivalries between nations, motivate their recruits with more joy anyone else can ever achieve, or warp fate itself.

Chaos does bring freedom and truth to the worlds it corrupts. There's no arbitrary hierarchy, no withering taxation, no censorship and no indoctrination in a usual sense.

But it is, indeed, Chaos. You aren't obliged to worship the Four, but you are not entitled to protection from being sacrificed to them. You aren't a slave of an institution, but anyone may enslave anyone by their personal power as they wish. You can, relatively easily, get immortality from Nurgle, magical and/or social power from Tzeentch, revenge from Khorne, and pleasure from Slaanesh. But anyone else can too, and you can become Nurgle's involontuary carrier, a pawn of a Tzeentchian, a Slaaneshi's plaything, or just another skull in Khorne's ever-growing pile.

It's not ALL total misery-mongering and doom, as you might think from looking at their army. Armies are naturally more horrifying than the civillians behind them. There are real, and pretty big, perks.

It's still pretty fucked up though.

The infighting of Chaos is less relentlessly brutal than the infighting of the Dark Eldar, but it is just as fractal and omnipresent. Gods plot against each other in their Great Game, worshippers pf one God fight worshippers of another in the name of their God, worshippers of the same God fight each other in their own name. As per legacy of Khorne as the first Chaos God, and as per eternal conflicts in the Galaxy, Chaos Gods are slightly tinted towards enjoying conflict within their ranks.

Chaos favors strong emotions. Drama and power struggle and war are definitely effective ways to obtain that. And so is fervent religious worship. But so are wild celebrations and festivals, and imaginative pieces of art. There are chemicals that achieve such effects more directly. It's not actually in the interests of Chaos to kill you - quite the opposite, it is in it's interests to make you feel more alive.

After death, the souls of the servants of the Four get into their domains as their property. In general, the Deep Warp can be concisely described as Hell. But for some, it can absolutely be heaven. It's a matter of personal taste and perspective.

So, Chaos is wide-reaching, powerful, multifaceted, quickly spreading, and all-around terrifying. Why haven't it conquered everyone already?

Well, there are three reasons for it. One is constant infighting on every level, from Khornates killing random comrades after running out of enemies to Tzeentch himself deciding he wants to sabotage the efforts of the Four today.

The other is the freedom and anarchy it brings. Chaos Gods reward worship, but largely leave their people free to "do as thou wilt", and governmental structures rarely last under their domain. There are no mechanisms for training and recruitment and supplying of armies for Black Crusades beyond shanghaiing, pillaging, and... asking nicely. There are ample opportunities for desertion and no one to hunt deserters down. If a Daemon Prince wants to, instead of conquering the Galaxy, meditate in the Warp for thousands of years or rule their Daemon World in peace, as most of them do, it's perfectly fine.

The Chaos Gods are by no means voluntarist, fully condoning murder and slavery and considering everything fair play when it comes to getting more worshippers, but they have their own weird sense of dignity and right to self-determination. They have come to define themselves in part as opposites of the Emperor and Imperium. Their most fervent worshippers defected from oppression and disciplinarianism, wishing for the opposite, and Chaos Gods, being what they are, obliged, to an extent.

The third reason is that everyone fears and hates Chaos. To some extent it empowers it, expectations of danger making it more dangerous. But in practical day-to-day matters, it's a hell of a handicap. Most people would rather ally with Necrons or Genestealer Cults than with Chaos. Most people would die rather than risk succumbing to it. Everyone is fighting everyone, but everyone is united in their fight against specifically Chaos.

And Tyranids, but you get my point.

Speaking of Tyranids.

They are an all-devouring horde of bugs, each feeling a constant, tortured hunger. They are a semi-sentient hive mind, so noisy as to blot out the Warp with their presence. They are invaders from far, far away who strip entire galaxies bare of oxygen and carbon. They add the biological distinctiveness of everyone they encounter into their own, and they modify their genetic makeup on their own to adapt to changing circumstances. Their infiltrators interbreed with anyone they find (yes, Tyranids are just that good at genetics) and create entire networks of hybrid-cults. Their Warp-enhanced claws and unbelievably efficient muscles shred Space Marine armor. Their immune systems cheerily tell Grandfather Nurgle "lol, you tried".