Dead things dreaming in the darkness.
Nothing stirred in the utter blackness of the Deepknell Depths. Nothing living remained in the vast catacombs and narrow tunnels to make a sound. The last incidental collapse of rock, or old bones finally falling apart to clatter across the coarse ground, had happened decades ago. No noise sounded in a place where nothing happened, and nothing ever happened down in the dark. So they had accepted a long time ago, that it was better to simply try to stop existing at all than to wrack and rage at the injustice of their fate. The people above had chosen to spend their lives as coin, and discard them when it suited them.
Cold and forgotten.
Those people had long since passed into the dirt, their progeny and their progeny after them living quiet and comfortable high above the erstwhile crypt. So quiet not even the slightest sound carried down into the dark. A peaceful life built on the suffering of thousands, ironically mounted above their very tomb. No passage down to the Depths existed, no crack for air to pass through or natural current of water. Noxious gasses were commonplace in the tighter tunnels, but they were oddly repellent to flame rather than conductive - early attempts at clearing the Depths of it's vermin infestation had involved the hurling braziers of flame both natural and otherwise down, achieving nothing more than the odd short-lived fire and scorched whisker, par for the course with the Skaven regardless.
Choking throats long-since rotted away.
When it became clear that the only solution was to confront the beasts in their own tunnels, hard choices needed to be made. The people of Deepknell Rock were largely peaceful, and were forced to petition the Stormcast Eternals, newfound defenders of Aqshy, to assist them lest their city and the Realmgate within fall to the insidious corruption of the Skaven. When the lightning-men arrived however they were not greeted by the familiar colors and heraldry of the Hammers of Sigmar, but the stark white and sky-blue of the Knights Excelsior. The Knights were a very different breed to their golden armored brethren, and immediately set to work. Forbidden from leveling the whole mountain, caverns and all, due to the presence of the Deepknell Realmgate, they came up with an elegant solution. The city had long housed an impressive prison complex, serving as the jailing quarters for the whole region. The prisoners within were offered a choice by the Knight's leader Cerrus Sentanus, they could die in their cells or take up arms and head into the caverns to redeem themselves and, if they emerged, win their freedom.
A choice between death and worse is no choice at all.
The people of the city had cheered as they marched into the Breach, and despite their situation the First Penal Legion of Deepknell felt themselves swell with pride. They had been dealt a hard hand, but one they had aided in drawing themselves, and those that lived through this would be welcomed back into a grateful city. They lit their lanterns, straightened their backs, and marched down into the dark.
Warp-lightning flashes searing cavern walls.
The Skaven are a perfidious race, and deep in the Depths they'd had nothing but time to prepare. Blasts of acidic green lit the caverns as engines of death lanced hellish energy through the ranks of the Penal Legion, with every strike claiming half a dozen lives. Neither force was well trained in the arts of war, and in that dank and dark battleground they fell to disorder, fighting as back-alley cutthroats in the dying light as the lanterns faded and the warp-lightning engines fell to the flaws of their own design. The last light to go out in the dark was the only natural source, the light filtering in from the Deepknell Breach entrance. There was no longer a way out.
A prison, traded unknowingly for a grave.
The bitter irony was not lost on the former prisoners. Sentanus would keep his word, freedom awaited any of the men that emerged from the caverns. It seemed he had simply ensured that promise could not backfire. Runes of sealing had been inscribed into the now-sealed entrance, a potent force that would ensure the Skaven Gnawholes present in the structure offered them no escape. Sentanus had cleared house effectively, dealing with both the Skaven infestation and the criminals his Stormhost harbored such contempt for. With the natural airflow cut off entirely by the enchantments of the Knights Excelsior the damned criminals and Skaven alike died by degrees as their own panic rapidly consumed the available oxygen. Within hours, every soul under the mountain lay dead, their throats clawed to bitter ruin by their own hand.
The rope would have been cleaner.
Within weeks the people of Deepknell Rock had stopped mentioning the prison that now stood empty at the edge of the city.
An axe would have been kinder.
Within years the last insect had died, having gorged itself on the last remaining flesh of the last corpse.
The price paid by the guilty forgotten by the innocent.
Within decades the First Penal Legion was forgotten. The Knights Excelsior had moved on to less remote climes, and the people of Deepknell Rock had used the breaking of Sigmar's Tempest as an opportunity to build a bustling hub at the edge of the realm. Aqshy was not hospitable by definition, but they had made a life among the geysers of boiling steam and hissing rocks. And far beneath them, the dead granted a mausoleum of their own by the cruelty of the Stormcast, had slipped into languid dreams to cope with the passage of eternity in the dark. Until, on an unusually quiet night both above and below the rock after a season of dark tribulations unprecedented in the fiery realm, a bow-wave of amethystine energy rippled through the world and set hot skin to cold sweat.
A thousand pairs of blue eyes lit up in the dark, the first light the cavern walls had known in decades. Scraps of bone swathed in ethereal blue mist seemingly pulled from the wave of energy itself lifted from the ground, dark animus hoisting the ruined corpses like marionettes on strings of cast-iron chain. As one they looked to their warden, the only soul beneath the mountain who had not come from a cell. Silent as the grave, as soft as the breeze who's touch they had forgotten, he pointed up through the rock to where a sleepy city was coming awake in surprise.