Skall: Skaven meets ratling meets Hrud.

So I had this idea. This horrible...horrible idea.

You ever meet that guy who's a complete nihilist, who thinks that all human beings are nothing but some malign blight upon the earth, no better than sentient viruses?

You ever think about what would happen if you made an entire race of those guys, and they just happened to epitomize everything they themselves said about the human race? Then, what would happen if you went beyond that, and you think about what it would be like to turn this idea up to 11?

Congratulations. You've just met the Skall.

It started when some ratling twists fled into the bilge pipes of a supercarrier shortly after the Horus Heresy. They were afraid. They knew the God-Emperor. They knew his divine light. But they didn't want to die, to be hunted down like vermin. They still wanted to serve. So the fled into the dark, still worshipping the icons they knew, despite their mutations. They kept the faith alive, feeding on the vermin of the bilge tunnels to survive.

Then they began to change. They evolved.

Millennium later, the Skall emerge from an unkown planet. They are small things that enjoy the dark, fleeing the light wherever they find it. They war among themselves. Why do they war? Not for purposes of faith. Not because of socioeconomic differences, nor political gain or ideological reasons. In fact, the entire species agrees on pretty much everything. So why the wars?

Because there are too damn many of them. They breed like wildfire, and they have no way of controlling it. Each one is a hermaphroditic breeding machine, and wherever a skall goes, it plants it's eggs whether it wants to or not. The eggs hatch into larvea, the larva grow into skall. The Skall have found no way to control this, because every day is a fight for precious resources. Every skall is too busy to do anything but secure meager crumbs of food to keep from starving, secure pitiful mouthfuls of water to slake their endless thirst, and killing their brethren to keep from being killed first.

The wars are simple things of population control, because without them the Skall would outgrow their resources and die. The skall are all too painfully aware of this. They make friends knowing that they may have to stab that friend tomorrow. They treasure friendship while it lasts before it is broken by inevitable betrayal. Each skall owns only what he can keep for himself. In the mind of the Skall, if you can't hold on to an item, then you don't deserve to have it. There are no property rights. There are no laws. There is only survival.

But all Skall know about what they had been once. They know that they weren't always like this. They served an Emperor who commanded from a Golden Throne. They sing of these lost times. They speak of them reverently. Indeed, the great unifying factor in the anarchic Skall society is they had holy purpose before their descent into depravity, their society had once been a thing of glory, a thing of greatness.

Obviously, it was only a matter of time before somebody happened upon these wayward abhumans. They saw their legends, saw their artifacts. They looked upon the crashed Imperial Carrier that was the birth of the Skall race, and they quickly deduced what had happened. And they saw potential.

Fast forward again. The skall now live in the underhives of every imperial city, they infest the bilge tunnels of every imperial ship. Their symbiotic relationship with the imperium is a strange one: They fight for the imperium, but they also invite the imperium to try and kill them off whenever their numbers swell out of control.

For the Skall, the Great Crusade never ended, nor will it ever, for the Skall procreate too rapidly for peace to exist. Billions of skall are dumped into crusades. More than anything, this is to rid the imperium of the Skall themselves, who would consume every last scrap of food and drink every drop of water in the imperium if it were otherwise.

The Skall know that if the Imperium wins, then the next step is a war between them and the rest of the Humanity, for only one can survive. And they dearly hope the imperium wins that engagement, for the Skall have lingered too long after their fall from grace. They serve now as they were always intended to serve: to secure humanities dominance over the stars. Tomorrow, their service will end when the Imperium dies and the Skall die with it, or when the imperium prevails and the skall must be wiped out. Either way, they will serve, as they ever have.

APPEARANCE:
To the Skall, their own appearance is a shame and a disgrace to humanities blessed form, and they keep themselves covered whenever they may. But caught without a robe, and one will behold a diminutive, three-eyed creature. They were once ratlings, but no longer. An extra eye has grown on the right side of their face above the traditional one. Their skin is oily black, and what remains of their hair comes in segmented strings, like an insect's antenna . Their mouths are now filled with rows upon rows of jagged teeth, much like that of a tiger shark. Their tongues are proboscis, as if they had replaced the organ with a lamprey eel. Their arms and backs are covered in quills of crystalized toxins, natural depositories for poisonous elements that the Skall biology cannot break down and consume otherwise.

TECH:
The skall are mostly scavengers of Rak'Gol-level tech. They still utilize Nuclear Fission reactors, and have developed a specialize splinter rifle that fires the quills that the Skall naturally produce. More fluff needed on their tech.

Note: They're not gonna stop fighting for self-preservation...ever. So they'll invite Space Marines to purge the hives, but the space marines really aught to be carefull doing so. Get isolated from the rest of your unit and you're suddenly swarmed by disgusting little creatures intent on eating you. The Imperium and the Skall would have to go to war eventually, it's just a matter of when. No Skall would ever say "Please, Purge me!" and offer himself for death. Rather, he would say "Come TRY to purge me!" Then he would start running. Then he would lay an ambush. And you better hope you win, because even if he's your best friend, he knows it's you or him. And he's determined not to let it be him.