The Dead Vault

In central Casmaron, in the heart of the Whispering Wastes, the yawning Pit of Gormuz stands as a reminder of past tragedy, ancient triumph, and a looming threat to Golarion’s future.
During the world’s dim prehistory, Golarion’s gods banded together to rid themselves of Rovagug, a deity dedicated to the destruction of all things the gods had set into motion. Through an alliance of necessity between Sarenrae and Asmodeus, who were aided by more than a dozen other divinities, Rovagug and his servitors were trapped within a demiplane thrust down through Golarion’s crust—buried and sealed, it was said, at the planet’s very core. The Pit of Gormuz descends down and out of sight, burrowing through the planet’s crust and plunging through each layer of Golarion’s Darklands. The massive shaft provides a direct path to the surface for their denizens, while simultaneously threatening them with the leaking influence of Rovagug’s fury.
The demiplane, known ever-after as the Dead Vault, is a thing of awesome power. Unlike many demiplanes, its exterior is crafted and defined, combining Sarenrae’s burning fury and the dark pragmatism of Asmodeus. It appears as a massive globe of cut and faceted golden topaz, a chunk of yellow stone fossilized around a deific insect, with rings of black iron coiling around its periphery and embossed with a terrible litany of binding runes. It sits lodged within the planet’s core, visible to any capable of pushing back the surrounding magma, but curiously untouchable. Any person attempting to physically reach the demiplane’s exterior experiences a strange warping of space that pushes the offending party away from the apparent physical boundary. Rarely, such an attempt may be successful, but only insomuch as Asmodeus sees fit to reward such success and perseverance by causing the Vault to swallow the intruder whole, trapping him inside as well.
Though such unfortunates are indeed imprisoned, the seals that hold Rovagug were crafted in haste, and the gods did not yet truly understand just what Rovagug was, nor of what the Rough Beast was capable. As such, the seals are imperfect, and cannot fully contain his power. Like a great sieve capable of holding back certain things while letting through those of finer size, the barriers block Rovagug’s escape but cannot fully prevent its influence from leaking out into the surrounding Darklands, eventually gaining in potency to the point where they trigger a monstrous, transformative malignancy within one of the Darklands’ natives, creating a monster and infusing its tortured spirit with that of one of Rovagug’s servitors.
This influence first came to a head in –3923 ar, giving birth to the first Spawn of Rovagug, and over the next 5,000 years the cyclic eruption of horrors continued to issue forth from the lip of the Pit, with such beasts as the Tarrasque— also known as the Armageddon Engine—and its kindred Vulnagur, the great beetle Ulunat, Chemnosit the Monarch Worm, Xonati the Fire-Bleeder, and others emerging to rain down the wrath of their “father” upon the face of the world.
Many have wondered about the Vault’s interior; as no divine servitors of Rovagug encased within have ever escaped, only dreams and visions sent by the imprisoned god provide a clue. Much larger than the 20-mile-wide lip of the Pit of Gormuz, the Dead Vault resembles a gigantic hollow world, thousands of miles in diameter, utterly devoid of light, and literally swimming with a chitinous ocean of the Rough Beast’s frenzied, self-cannibalizing servitors. But other things exist: entire sections of the Darklands have been subsumed and absorbed into the vault, especially parts of the lowest layer and the bizarre creations of the Vault Keepers, as well as packs of devils and others that somehow survive amid the devouring anarchy.
These last prisoners raise yet more questions: Has Asmodeus used the Vault as a repository for more than just Rovagug? Does the god-fiend exercise his power over its gates to condemn enemies to a certain, swift death or eternal silence trapped within a realm to which only he possesses the key?