Castle Void was a truly obscene and unnatural place, or it would be according to the natural laws of just about any Dimension Çrom had ever heard of. Since the Void was a universe possessed of properties that made the Darking construction just barely possible, however, it was pointless to go around complaining about how the whole thing was an abomination unto physics.
So yes, it was obscene. But it just scraped through on ‘natural’. Just. Mostly.
A lot of now-forbidden magecraft and dark science had gone into the construction of Castle Void, as well as megaengineering in its purest, most audacious, and perhaps even original form. It was, from Rooftop to Darkynvault, a built thing, not a natural phenomenon. A gloomy labyrinth of grey stone somewhat larger than an average observable cosmos didn’t just happen, even in a place where the prevailing laws of time, space and matter seemed custom-fitted to said structure’s formation. The Darking God and Disciples had forged their ridiculous home out of the vast floating stones of Castle space using arts that strained even the Void’s permissive standards.
The Darkings were a Firstmade Brotherhood, and Firstmades had their own rules about what they were allowed to do with the worlds, mortals, even entire Dimensions of which they assumed ownership. They’d written the first rulebooks, after all, and overseen the writing of all the subsequent rulebooks, so it was easy to work in the occasional clause or loophole or fine print. Some of the greatest and most bizarre of their achievements had been made when the urverse was new and the Corporation did not exist, and even the Ghååla were still scratching Their heads and deciding what should and should not be allowed in a sane reality. There had been no Relth to police the halls of invention. Limbo had stood alone in defence of the integrity of the urverse, and was known to be amenable to persuasion if you promised to name a drink after Him.
Before mortals had arrived and begun to push and pick at the edges of the possible, the Firstmades had long since strode out far beyond those edges, put their hands on their hips, and said I think I will write my name in diamonds the size of galaxies right here.
The Darkings had come to the Void Dimension long before the Pinians, and had been building ever since. By the time the Pinian God arrived on the Face of the Deep and bade there be light, what that light mostly illuminated was a fuckalmighty huge expanse of grey stone roof. And one of the things the Darkings had harnessed, to convert the floating stones of the Void’s lower reaches into the unholy edifice from which they ruled, was the Voidstuff.
It had been more vital, once. Equal parts destroyer and creator, a product of magic both mortal and Divine, field generation and matter reconfiguration technology melded with a decentralised infusing quasi-sentience that was frowned upon even before the conception of the Elder Races. In Castle space its existence was tolerated for longer, under Firstmade-age stipulations and due to the fact that it was a long time before any power could really hope to rival the Darkings in their vast, gloomy, fortified home.
These days Voidstuff was classified as a native non-sentient. It no longer did its work, tearing apart world-sized agglomerations of Castle space stone and churning them into chambers and plates, buttresses and columns. Somewhere, far away on the edges of the Castle Void construction, it was possible that tendrils of Voidstuff still toiled away at the task for which it was created. It was plausibly deniable, because the Darkings had more convenient construction methods at their disposal these days. And the wave front of the building expansion was simply too far away to enforce the ban against nulliform quasisents in any case. The distances involved made it practically impossible for lesser beings to even traverse them in a single lifetime, and – frankly, once you got out there you’d still have the invincible and timeless monolith of Firstmade legal precedent to contend with. Not to mention their functionally infinite wealth and authority.
In Castle Void proper, the Voidstuff was little more than a technowraith. A sad and lonely near-entity, bereft of purpose. It roamed the halls, waiting for orders it was no longer permitted to be given.
Until, sometimes, it was given them.
It was difficult to see Voidstuff when it was just moving from place to place. It was visible mostly from its effect on the stone. The blocks and slabs and rough-hewn grey surfaces contracted and rippled, but only by a matter of microns. You could feel it more as a shift in the air and a strange sense of disquiet than an actual visual or tangible shift. It was practically invisible, until it struck.
Çrom had heard stories, almost as long as he’d lived in the Four Realms, of rogue Voidstuff climbing the Eden Road and working its way into the stones and structures of the Four Realms. Claustrophobes, it was said, were simply people who were more finely attuned to the silent presence of Voidstuff, could feel it gathering in the walls of enclosed places, could sense its desire to crush and smother and destroy.
Ludicrous, obviously. Voidstuff could not function outside of Castle space. That’s what it was made for. Of course, the Four Realms were built in the Face of the Deep, on the cusp of physical laws between Castle space and stellar vacuum. It was possible that Voidstuff could seep into the matter that made up the flatworlds … but it would be powerless, an impotent but disturbing presence, incapable of action.
 Or half a universe, to be entirely technically accurate about it.