A once great and powerful empire in Casmaron until it was decimated by the greatest Spawn of Rovagug, the Tarrasque. Its once-grand structures are now little more than dust-choked ruins, and its people only wandering ghosts.
Geographically, located at the heart of the Casmaron continent, on the western shore of the Castrovin Sea. Among the mightiest empires ever known on the face of Golarion. A very militaristic nation, and its armies at one point had conquered lands from all across Taldor in Avistan, most of Casmaron, and even over into Osirion in Garund.
In Chapter 186, Professor Rupin shared a piece of Ninshaburian history with Pandora and Tharivol:
It all started back in the Age of Creation. Picture that period of history: gods walking the surface of Golarion, yes. But also gods rampaging and destroying. The Windsong Testaments tell us that no sooner had mortal life been created, than Rovagug The Destroyer gnawed his way out of the Rifts, through the Astral Plane, and into the Universe. He immediately consumed seven planets, but since they had no names nor histories—nor any true Potential—their taste was dull. He hungered for more. And so it was his attention turned to the great triad of planets named in the Testaments as Earth, Androffa, and Golarion. Three planets whose history was young, but whose names had the potential to ring through the eons as hubs of consequence and power. We know not whether Earth or Androffa survived his assault (indeed, whether or not they even truly existed), but we do know what legend says happened here.
Sarenrae gathered under her banner the most unlikely collection of allies—gods of good and gods of evil—and they defeated (but did not slay) the Rough Beast in a battle at a site that would come to be known as Gormuz.
Sarenrae told the fledgling mortals of that age to avoid the site. But as humanity rose in Casmaron, they found Gormuz and they felt—but did not understand—the power radiating from the site. They built a city there, which would become one of the largest in the world. And civilization grew out from there, growing eventually into this Empire known as Ninshabur.
But not only were mortals drawn to Gormuz, so too was the herald of Rovagug: the Tarrasque, and around 5,000 years ago the Tarrasque arrived at Gormuz, and there began a rampage that wiped the empire of Ninshabur from the face of the planet, and ground its entire civilization into dust. Half of the continent of Casmaron was completely destroyed. Many scholars believe that before the Tarrasque there was no Windswept Waste. The Whistling Plains were likely hundreds of miles of great cities, now lost. The Meraz Desert was once likely fertile farmland, now empty waste. Some believe the terrible Parchlands may have actually preceded the time of the Tarrasque, but we’ll never know for certain.
All that survived the destruction of Ninshabur was what little was left on other continents, in the places where their armies had once passed. Small personal trinkets. Slim, barely legible scraps of parchment.
First Reference: Act III Prologue
Other Notable References: Chapter 186