Predictably, as the Ratlings of Ornsworld tremulously began the construction of the Great Spire under the Planetary Governor's decree, the verminous pests ceased their predations. They no longer assaulted the laborers nor devoured their meager rations. In stark contrast, those who refused to toil upon the monument suffered twofold; families were discovered in their hovels, gnawed down to skeletal remains by the swarming rats. Initial waves of horror soon gave way to a chilling numbness. Survivors began to scavenge the estates of the deceased before the bodies were even cold, their desperation overriding all morality.
Driven by fear and the instinct for survival, an ever-increasing multitude of these half-height abhumans joined the labor. Together, they raised a jagged, sky-piercing monolith that clawed at the firmament!
Only the final task remained. Under the feverish, devout gaze of the Ratling masses, a massive bell, cast from pure, unrefined Warpstone, was hoisted level by level up the spire.
The heavens churned with bruised clouds, split by staccato bursts of viridian lightning. A torrential, pitch-black rain lashed the world. Bilbo Battis, Planetary Governor of Ornsworld and self-proclaimed King of the Ratlings, sat atop his palanquin. His servants strained under the weight of his bloated form as his eyes remained fixed, with obsessive intensity, upon the beautiful, sickly green luminescence radiating from the bell.
After thirteen days and thirteen nights of grueling labor, the bell was finally seated upon the highest spire of the world. Every Ratling on Ornsworld fell to their knees in frantic prayer. Without even realizing it, they had long since abandoned their futile pleas to the God-Emperor; for those who had most fervently invoked His name had been the first to be devoured by the vermin.
As Bilbo Battis stared at the bell, the figure of the black-robed stranger flickered into existence before the Warpstone mass. Above, the sky curdled into a suffocating shroud of darkness, the green lightning weaving into a terrifying, electrified tapestry.
Then, the heavens seemed to collapse. A horned rat-skull of titanic proportions, marked with the triangular sigil of the Great Horned Rat upon its brow, manifested in the clouds to stare down at the huddled masses below.
The bell tolled.
The sound was deep, abyssal, vibrating not through the air but through the very souls of those present.
The Ratlings trembled with primal dread. Bilbo Battis tumbled from his palanquin, his flabby face quivering as a name clawed its way out of his throat: "The... Father Below! The King of Many Tails...!"
Like a viral contagion, the chant spread. Thousands upon thousands prostrated themselves, shrieking the titles of the Father Below and the King of Many Tails. The monstrous rats surged from the sewers once more, but they did not attack. Instead, they recoiled into submissive hunches, chittering in unholy worship of the great skull looming in the sky.
The tolling of the bell and the cacophony of chittering grew in volume until the entire planet vibrated with the sound. As the thirteenth peal echoed across the world, every voice cried out in a singular, horrific unison:
"The Great Horned Rat walks among us...!"
With the final word, a violent metamorphosis seized both abhuman and vermin alike.
Fur sprouted from their skin; ears elongated and twitched; long, whip-like tails erupted and lashed the air. Their metabolic rates spiked to frenetic levels, accelerating their already cunning minds into a blur of hyper-active thought. Agonized shrieks tore through the air as the transformation took hold.
Ornsworld had fallen. The Ratlings were no more; in their place, the Skaven had claimed a new domain within the heart of the Imperium.
Lucius watched the kneeling throngs with dark satisfaction as tides of faith and raw emotion surged into his empyrean essence.
"Ah... I am becoming... whole!" Lucius exhaled, stretching his divine form as the powers of treachery, despair, and baseness were formally woven into his mantle.
He spoke, his voice echoing in the minds of the transformed: "From this moment forth… you are Clan Ratling. One of the Council of Thirteen within the City of Blight. Bilbo Battis, from this day you are Battis Swollentooth, Warlord of Clan Ratling."
The former Governor, now the bloated chieftain of the clan, hauled his mutated body from the filth. His beady rat-eyes gleamed with newfound avarice and treachery. "Yes-yes! Praise the Father Below! Praise the Great Horned Rat!"
Simultaneously, across the galaxy, other Ratling populations began to undergo the same inevitable corruption. Fur and tails sprouted; incisors sharpened; their natural greed curdled into something far more sinister.
The Astra Militarum was plunged into immediate chaos. As vital scouts and snipers, the sudden mutation of the Ratlings sparked widespread riots and purges within the regiments. The Ratlings deserted en masse, vanishing into the bowels of hive cities and trench systems. The Imperial Guard pursued these "mutants" with righteous fury, but those who followed them into the dark tunnels never returned.
"How is this possible? Why have the Ratlings literally turned into rats?"
Lord Generals and Commissars stood over the dissected remains of captured specimens in utter confusion.
"My Lord, their physiology remains humanoid but is radically altered," a medicae-biologis explained, gesturing to the specimen. "Their metabolism has been overclocked. They are swifter and more agile than any human, but their lifespans have plummeted as a result."
The Lord General exhaled a cloud of promethium-scented smoke. "I care not for their biology. They are specialists in reconnaissance; do not waste our strength hunting them in the dark. Seal the sublevel access points. Let the vermin starve in their holes."
Under the fragmented and bureaucratic madness of the Imperium, these outbreaks were treated as isolated incidents. Commanders saw no need to coordinate, simply planning to requisition more "fresh" Ratlings from Ornsworld once the warp storms subsided, unaware that their source was already gone.
However, on a world within the Imperium Nihilus...
Lion El'Jonson, Primarch of the Dark Angels, stood amidst the ruins of a defeated Ork horde. He had led his "Fallen" Risen and the survivors of Bard to cleanse the world, but his heart burned with a cold, humiliated fury.
To have been bested by a rabble of unknown rat-xenos was a stain the Lion could not tolerate. He had sworn to exterminate the foul species and wash away the shame of his Legion. Yet, before he could find their trail, the Ratlings stationed on his current world had mutated and fled.
The Dark Angels had been swift, capturing several of the fleeing creatures. When the Lion gazed upon the twisted, rat-like captives, his initial rage was tempered by a chilling realization.
He felt the unmistakable taint of Chaos clinging to their fur. He knew these soldiers had been loyal warriors moments before their sudden, unnatural corruption.
A dark thought took root in the Primarch's mind.
"Perhaps," the Lion murmured, staring at the chittering things, "my enemy is not merely another xenos threat to be put to the sword."