Neath the skin of the world, in the dry capillaries of mountains' dead bones, lie black gates to Naussia.

Naussia, once a realm of august sorcery—neither monarchy nor coven,‡ but lord apparent to all the world—is now a relic. An antiquity black with wicked tarnish, sunk and sleeping; fled to the foreign depths of the Underworld.

At their height, some two thousand years ago, the armored sorcerers of Naussia commanded all the mountainous spine of the world. ‡‡

From catacomb fortresses, cut miles into granite by mindless slaves, they brewed armies from gluttonous cauldrons: Thousands of lobotomized slaves, eyes shut by steel plate, labors prolonged by sorcerous half-life. Countless chimeras cut from scorpion and bull; babe and wolf; serpent, goat, and lion. ‡‡‡  Millions of cauldron-born grues; plague repurposed with military intent. With these armies, the Naussians conquered all the world, fast and confident in their lairs.

No foe endured before the Naussians, for no human force could endure the ravages of plague and endless attrition. With every cauldron-born slain, another corpse was fed to the pots of sorcery, soon ready for redeployment with new life and fresh armament. Attrition assured the Naussians every victory, for their reserves grew with every death.

And all the while, their sorcery grew only more terrible. In their formative years, they quick slipped the limits of death, became no longer men and women, but beings of their own design: Creatures of high, terrible elegance. They forsook their mortal bones for those of titans, greater and more statuesque. They flayed their own frames, reset the flesh with plated, ebon iron. They discarded their own faces, their human eyes, chose each instead the blank, beaked gaze of high terror. §

The Naussians cared not for low Humanity, let alone what remained of their own. Any and all sorcerer-comrades to perish in the name of domination and hideous progress would be gladly dissected, absutured into incunabula of immense scale. These were books of sorcery, sewn from the brains of the greatest, most perverse practitioners to have ever crafted the black art. By these fleshly tomes, the knowledge and culture of Naussia grew ever greater, ever queerer with every passing century. §§

But before the third century of its height could round, Naussia was met with one foe which would not be so easily quashed nor ensorceled: The Otherworld, and all its fickle children.

For all those brief centuries after the interstiction of the worlds, the Naussians had ruled in a relative vacuum, contested only by peoples of middling power and small sorcery. After crushing these, they set their dire conquest to the world's frayed edge: The raw borders of the alien Otherworld, the home of älves. Black eyes in creeping mist, capable of sudden and vicious violence when provoked.

The Naussians' provocation proved their undoing. Though they had no iron, no Worldly sorcery, the älves' response was fast and devastating. Nigh-invisible, immune to plague, and charged with a hostility so honed, so coordinated, it could only be the product of an ecosystem itself enraged, the pale soldiers of the Other cut down cauldron-born armies with one-sided ease. Sorcerers themselves fell on the battlegrounds, their iron carapaces proofless to the cruel ribbon-lances of Otherworldly knights.

Whether by fear or by hard consensus, the Naussians withdrew to their fortress catacombs. To the black of the Underworld§§§ they went, to their square-kilometers of chiseled halls in lightless granite, carved by millions of cauldron-slaves over an age of dominion.

In that awful subterranea, in uncounted, disparate tombs of unfathomable scale and separation, the Naussians buried themselves.

They locked away their cauldrons, their precious tomes in trapped complexes of horrid device, guarded by their direst, deathless chimeras. They marched their remaining legions into bonepits, ossuaries for once and future armies. They hooked their legions of slaves by their collars and their thin hips, hung them by the thousands in stark halls, like queer stalactites. They interred even themselves, in black sarcophagi secreted in grim sepulchres. 

In this way, Naussia disappeared, came to rest neath the skin of Nören for ponderous millenia. It has slept, in all its strength and all its sorcery, in the silence and the dark, unto now. Unto the ripening of the world.

Somewhere, after all this time, black cauldrons are lit anew.

As a world element, Naussia facilitates ludicrous, Mines of Moria-level negadungeons.

To stumble into the Underworld is already fearful. To fall further into a Naussian tomb is a mishap worthy of real dread. As I've run it, it leads to a session or more of panicked escape, wherein one or more characters often die. Those who escape often do so with little to show, save tales of dread and dubious artifacts better left under the earth. That, and a good hunk of XP under their belts. 

To go willingly into the home of Naussia is to tempt doom. These are delves of highest risk, often justified by the acquisition of a single tome within. That, or meat grinder raids cruelly organized by banks. More on this to come.

Footnotes

* The West Garden, a gated and forbidden portion of the Royal Academy grounds, plays host to
weekly meeting of students interested in more than simple explorations of arcana and occultism. There, secrets, art, and lust are traded over smuggled cups of wine. Älves are rumored to attend.
** The concept of Manifest Doom dictates that the Coast, without intervention, will be subsumed by the encroaching borders of the wilderness, and, likewise, the Otherworld. "Imperative destiny" is an accompanying philosophy. It dictates that expansion into wilderness is both necessary and inevitable.
*** "Anteinterstiction" refers to a time, some 3,000 years ago, wherein the world was reduced to a tiny scrap spliced chaotically by its neighbors: The Coast.

† The Ancient Nör were an anteinterstiction race of humanity. They were the first sorcerers, commanding arts more awful than any known after. By their hubris, they burned nearly all the world, creating the Coast and destroying their empire in the process. Their ruins and language shape much of modern society.
†† The Præcantian Age was a time of high sorcery in which many sorcerous empires rose and fell. They sprung up in the ruinous wake of the Ancient Nör, fueled by scavenged sorcery and savage times.
††† Throughout history, "sorcerous empires" or "sorcerer's empires" have prevailed as the most
awesome and terrible of all.

‡ The structure of Naussian command, what existed, is greatly unknown. From what can be gleaned by translated writing, which are sparse, as access to them comes at a price of danger, the Naussians were a near-scholarly alliance of several hundred sorcerers united by insular culture, aesthetic, and aims, each of whom shared their knowledge of the black art, thus allowing alliance-wide gains in sorcerous power.
‡‡ Naussia commanded the full extent of the gigantic Gorathian Range, a land East of the throne of the fallen Ancient Nör, whose powers they salvaged and reverse engineered. The coastal locations which surround said range, though powerful in their own right, are thought to have served tithes of slaves and materials to the Naussians.
‡‡‡ A chimera is any sorcerous hybrid created by cutting, brewing, and splicing disparate creatures into one monstrous product.

§ Naussian helms, or faces, as they may be, are oft described by archaeologists and cutters as akin to the skulls of birds. Each is distinct, despite this unifying motif.
§§ A few Naussian Tomes have been discovered my modern explorers. Each is a thing of true awe: An altar, half the size of a man, of fleshy pages bound in hinges and plates of thick, pitted iron and chain. Their contents, written in High Naussian, are apt to change, when read. They are impossible to write in, as other incunabula are. One, the Wicked Tome, is known to have been destroyed, its contents detonated atop Mount Hellebore. Another, the Vile Tome, is yet at large. It is rumored to be an item of immense hazard, as all who fall asleep in its vicinity are driven to a state of unconscious, perverse violence.

§§§ Naussian sorcerers' greatest gift, that for which they are named, is their prowess in the manipulation and voyaging of Underworld gates. This discovery granted them reliable access to the lightless world's depths, without fear of entrapment.