As the Skaven overran the elevator tiers of the Stygian Spires, Corrupting Engines were deployed in rapid succession. Their baleful influence bled into the spires' systems, saturating the machinery with pure Chaotic energy. These virulent energies swiftly defiled the intangible Machine Spirits, binding them into the servitude of the Great Horned Rat.
Once corrupted, the mechanical apparatuses became accomplices to the vermin-kin, turning their integrated weapon systems and logic-cores against the enemies of the Rat.
The moment the elevators reached the summit, an ocean of Skaven surged forth with piercing screeches. This tide of vermin rivaled an Ork Green Kroosade, with jade-green tracer fire flickering across the gloom.
Strangely, the Adeptus Mechanicus Skitarii failed to mount a cohesive defense. From behind the iron ramparts of the Machine God, the Skitarii's return fire was sporadic and disorganized, utterly lacking a unified tactical oversight.
In truth, the Skitarii were leaderless. The Archmagos, who should have been leading the tech-priests in the rites of battle-incense, was instead lost in frantic supplications to the Omnissiah, leaving the Skitarii Marshals with only localized, faltering control.
Weapon teams clad in Warp-Power Armor, lugging heavy ordnance barrels nearly as large as the Skaven themselves, scurried onto the firing line. A Skaven assistant, hunched under a massive ammo-pack, snatched a glowing green glass sphere and slammed it into the maw of a mortar.
"YES-YES! Ah-haha! Fire-launch! Kill-slay the iron-skin man-things!"
Thump-thump-thump—
The Poisoned Wind Mortars barked with heavy reports. Even encased in power armor, the Skaven staggered under the violent recoil. However, the range of these Warp-variants far exceeded their archaic ancestors from the World-That-Was. The mad scientists of Clan Skryre cared nothing for the ergonomics of the user; they sought only to squeeze every drop of lethality from both gear and crew.
From a distance of nearly four kilometers, the mortar shells descended upon the mechanical ramparts like a hail of emerald ice. The sound of shattering glass erupted, followed immediately by the rapid expansion of hyper-concentrated Warp-Toxin gas.
"Gas?"
A Skitarii Marshal, one of the few units possessing high-level autonomous thought on the front line, watched the green vapor approach with cold disdain. He considered the xenos fools; nearly every Skitarii under his command had been fully augmented. Biological toxins were useless against the sanctified cyborgs of Mars.
But his logic failed. One by one, the Skitarii collapsed, their power-packs seizing. When their crimson robes were torn away, it was revealed that their remaining flesh had been liquefied. Worse, even their bionic components suffered from catastrophic corrosion; cables and cold steel melted together in a sludge of rusted slag.
As the iron walls failed, Warp-Drill teams breached the foundations. Through the jagged rents, an uncountable swarm of Skaven poured into the sanctum of the Stygian Spires.
Rikcruk Sliceblade led his personal retinue of Stormvermin in a decapitation strike toward the spire's core. The hulking Skaven moved with unnatural speed, his blade-work a blur as he butchered the intervening Skitarii and even cleaved through three Sydonian Dragoons with singular, monstrous strikes.
"The iron-things inside! They are mine-mine! Sliceblade's!"
Rikcruk charged into the inner sanctum, eager to skin the enemy leader alive to bolster his own infamy. He was met with bitter disappointment.
The Archmagos and the high-ranking tech-priests were already dead. Their mangled corpses looked as though they had been chewed to pieces by the massive, rodent-shaped mechanical gargoyles adorning the architecture.
"CURSE YOU, SKRYRE!!" Rikcruk shrieked in fury.
Once a site was claimed by a Corrupting Engine, the environment itself became a predator. Much like the lands of the Mortal Realms in the Age of Sigmar, the soil, and here, the steel, became an extension of Chaos, attacking the forces of Order. Only the self-redeeming blood of the Flagellants could purify such filth, but here, there was only the Rat.
The Stygian Spires had become a Skaven hive.
Down at the base of the spire, Kratch Doomclaw received the report. He was ecstatic; by seizing the water source, they had secured a strategic stranglehold. Yet, the Skaven faced immense pressure as enemies began to converge from all directions.
Under the lash of the Clanrats, a sea of Slave-rats began constructing the vermin's first surface fortress. It rose like a mountain of illegal architecture—scrap piping, scavenged steel, and detritus piled tier upon tier in a ramshackle heap. The Skaven hive quickly swallowed the elegant, baroque designs of the Adeptus Mechanicus.
A massive, green-glowing Great Bell was hoisted to the highest peak. With its installation, the Skaven felt certain the Great Horned Rat's baleful gaze was upon them.
Indeed, the shift did not go unnoticed. Outside the Stygian Spires, the Ork hordes, previously held at bay by a coalition of Imperial Fists and Skitarii, realized the "flavor" of the battle had changed.
The scent of a real scrap guided them. These were Speed Freeks, and they gunned their engines. With a deafening roar, their ramshackle, high-octane war-bikes and kustom buggies, some boasting exhausts larger than a heavy hauler, thundered toward the spire's ground defenses.
Kratch Doomclaw watched the approaching dust cloud. Those high-displacement bikes and steamroller-like vehicles would shred ordinary Slave-rats like paper. Gritting his yellowed teeth, he barked an order:
"Go! Tell those... those cursed Skryre things that cost us so much warp-token! Send out the Flayers and the Doom-Wheels! Stop the green-things!!"
Kratch's lieutenant scurried away in a panic. Soon, from the twisted tunnels of the new hive, the screech of grinding metal and the roar of erratic engines answered the Orks.
The Orks squinted through the dust and let out a collective roar of joy. Emerging to meet them across the wasteland were hundreds of metal monowheels. The spinning blades at the front and the twin-linked lightning cannons mounted to the chassis made the Orks howl with excitement.
"Oi, listen up, ya gitz! Dis ain't no lil' stroll in da mud, dis is a proper race! Don't make da Boss look bad, ye git! Speed Freeks don't lose to no stinkin', twitchy grot-snots! Smash 'em, squish 'em, krump 'em good! Gimme a WAAAGH!!!—WAAAGH!!!"
With the Nob's roar, the Orks slammed their accelerators and charged headlong into the Skaven's engines of destruction.
…
"Lord Calgar, we have confirmation: the Stygian Spires have fallen to xenos forces," an Ultramarine reported to the Chapter Master.
Marneus Calgar's brow furrowed into a deep, stony scowl. The Stygian Spires provided the majority of the water for all of Vigilus. If they could not be retaken, the fighting capacity of the mortal defenders would collapse.
However, the Spires were nearly a third of a planet's circumference away. Even Calgar could not guarantee his forces would survive the trek through the endless layers of Ork-infested "Scrap-Cities" that choked the wastes.
Then, worse news arrived. The noble House Agamemnus had dispatched their private enforcers to seize the food and water from the lower-hive citizens. They had retreated into their family spires, abandoning the common people to their fate.
"The damnable parasites!" Calgar growled.
He knew the actions of these Imperial traitors would accelerate the decay of the Imperium's grip on the planet. Genestealer Cults and the touch of Chaos would spread like wildfire through the starving, desperate populace. Yet, as a Chapter Master of the Adeptus Astartes, his authority to execute planetary nobility was bound by complex webs of Imperial law.
"Increase patrol density in the Dirge Mast district. We cannot allow the xenos or the Archenemy to spread further. Any act of sedition or betrayal must be met immediately with the Emperor's Judgement!"