While war raged across the surface of Vigilus, the Skaven spread through the planet's crust like a galloping plague. Slave-Rats, wielding crude tools looted from the surface-dwellers, worked with frantic desperation to expand the scale of the Under-Empire.
Countless tunnels branched out from the primary nests in a claustrophobic web. Emboldened by their numbers, the Skaven began to raid the lower levels of the planetary hives with increasing audacity. Though rumors began to circulate among the hive-populace of upright, furred xenos stealing food-stores and dragging laborers into the dark, the authorities of Vigilus, besieged by Orks and the open rebellion of the Pauper Princes Genestealer Cult, had no time for such "fanciful" superstitions.
This ignorance was the fuel for the Skaven's proliferation.
Within the Breeding Pits, castrated rat-eunuchs moved silently, their maws gagged with iron bits to prevent them from cannibalizing the young. They tended to the Breeder Queens, mountainous mounds of pulsating flesh that served as the clan's biological engines.
"Squeak-chitter-squeak-squeak…"
The sound was a cacophony that would turn a mortal's stomach. In these vast, filthy chambers, flesh-pink pups as numerous as ants swarmed over the Queens, fighting for the teats that Clan Moulder's mutators had grafted across their entire distended abdomens.
The Queens were the ultimate sacrifices to the Under-Empire. The eunuchs placed massive vats of "slop", a revolting slurry of rotted meat and warp-elixirs, before them. The Queens, their heads unnaturally large and teeth yellowed with decay, fell upon the feed with primal hunger, their thrashing maws clanging against the iron troughs.
Meanwhile, the eunuchs sifted through the writhing carpet of pups. Every female was shoved into a burlap sack and carted off to the Mutation Chambers. There, they were subjected to immediate warp-chemical alteration. Their bodies would bloat to grotesque proportions, every nutrient diverted to their wombs and mammary glands until their limbs withered into useless bone-sticks. These "refined" Queens were then shipped to newly excavated pits to begin the cycle anew.
As for the males, if they survived the cannibalistic hunger of their litter-mates, they were raised on hormone-laced milk. Within three months, they reached combat maturity. Their place in the hierarchy was determined by the color of their fur.
The rare albino-furred pups, especially those born with budding horns, were taken by the Grey Seer conclaves to be indoctrinated in fanatical devotion to the Great Horned Rat and the manipulation of the Warp. Pure black-furred pups were sent to the Stormvermin barracks for a life of brutal martial discipline. For the rest, the line between Clanrat and Slave-Rat was a thin one, crossed easily by a single act of betrayal or a failed assignment.
The House Agamemnus, which shared planetary governance with the Adeptus Mechanicus, ignored the reports of missing workers. With Ork "Speed Freeks" battering the walls of the Dirge Mast daily, who cared for the grumblings of the dregs?
Irony, however, is a cruel master. The first to investigate the "upright rats" was not the Imperial authorities, but the Pauper Princes. Linked by the psychic tether of the Broodmind, the Genestealers were blinded by the shadow of the Hive Mind, but their internal kinship remained unbreakable.
From the sewers beneath the Greigan Hollow, now a Genestealer stronghold, the Cult organized patrols to find the cause of their disappearing kinsmen. As two species born for the shadows, the Genestealers were naturally suited to hunting the Skaven.
"Caution. Do not be complacent," warned a Nexos atop a Goliath Rockgrinder, using his tactical overview to scan the environment. Behind him, Neophyte Hybrids in stolen flak-vests clutched autoguns, their purple-tinged skin and bald pates glowing in the dim light. They lacked the discipline of the Astra Militarum, but in the dark, they knew no fear.
They moved through sewers a thousand years old, treading over ancient filth and nameless bones. Suddenly, the sound of scurrying footsteps echoed from behind. The Neophytes spun about, unleashing a deafening volley.
Tracer fire lit up the damp tunnels, ricocheting off ancient masonry.
"GYEE-AAUGH!"
A high-pitched shriek rang out, giving the Cultists a target.
"Enemy spotted! Attack!" the Nexos commanded, though he felt a flicker of confusion; his tactical senses had failed to predict that the enemy was already so close.
The Cultists rushed forward to find foul, dark blood splattered on the walls. A narrow fissure had been cut into the ancient stone—the enemy had been watching them from within the very walls.
Zip-zip-zip—
Without warning, streaks of emerald fire erupted from the darkness. The Warplock rounds punched through the heavy industrial hull of the Goliath Rockgrinder, bypassing the reinforced plating designed to withstand falling boulders and detonating the fuel tank in a roar of green flame.
As the vehicle exploded, the deathly silence of the sewers was shattered by a tide of noise.
"YES-YES! Kill-slay them! GO!"
A raspy, screeching voice commanded the darkness. A powerful Skaven Warlord, brandishing a glowing Warp-blade, directed a shoulder-to-shoulder mass of vermin as they poured from every shadow and drainpipe.
The Cultist commander barked orders, and the Neophytes formed a desperate perimeter around their remaining vehicles, returning fire to cover the retreat of their leaders. This time, the Skaven did not lead with a wave of slaves. Instead, Clanrats shouldered their Warp-muskets and engaged in a disciplined, if frantic, firefight.
The Skaven's accuracy was barely superior to an Ork's, but they had numbers. In a tunnel of less than a hundred square meters, the air was saturated with green warp-bullets. Within moments, the rearguard was annihilated. Only a few Cultists managed to escape, their Rockgrinders crushing everything in their path as they fled.
When news of the skirmish reached Kratch Doomclaw, he backhanded the commander in charge of the raid before his expression turned venomous.
"The hairless-things from above want to enter the sewers? They want the tunnels, Yes? Then the Skaven shall pour out... and drown them all!"
After months of silent growth, the Under-Empire could no longer be sustained by petty theft alone. Whether to cull the excess population or to secure more "meat" and territory, Clan Rictus was ready to officially join a war that was about to become much, much bloodier.