The Greys continue to be the popular choice as the creators of the Black City (73% of the vote). Their colony on Earth, the Black City, disappeared some time during the last period of glaciations (within the last 100,000 years); so what happened to the Black City after the death of the Greys?
Many of the slave races spawned in the Black City escaped into the wilds; these included early humans and antecedents of the humanoid species that still dwell in the dark places of the Earth. The city and its creators continued to surface in the myths and legends of the earliest civilizations; the city of the immortals, the city of the gods, the city of enslavement, the city of heaven.
It was in the time before the ice retreated, when it was possible to walk much of the earth that is now underwater, when early humans returned to the city, seeking to steal fire from the gods. They returned back to their homes with something much better - tablets and writings that laid out the workings of the magic still practiced today amongst the wizards of man. The glyphs and writings from the Black City were compiled into the first grimoire, and those earliest of magicians led their people to the founding of Hyperborea and the flourishing of that first civilization.
The Hyperboreans used the lore wrested from the Black City to discover sleeping gods, and they grew to worshipping the foul things of the earth, creating many blasphemous cults. Their story and the fall of the Hyperboreans is for another day, but their cities were turned to rubble by the retreating glaciers, and the legends of their civilization come down to us primarily through the myths and stories told by the Corinthians.
The retreat of the glaciers and the rise of the oceans also isolated the Black City, eventually trapping primitives and monsters alike in the cold wastes.
Others have discovered the city in the ice. Writings of the Acheronian historian Kluny refer to the missing ships of the Classis Nemea, the northern fleet; Commander Titus Plotius Nasica is attributed with leading trireme Minerva and other landing craft to find the lost land of Thule after learning about it from the conquered Hibernians. Although Minerva and her ships never returned to Acheron, 500 years later, the Northmen would find Acheronian artifacts and remains in the ruins of the Black City, leading one to believe the Nemean fleet did land on the island, once.
A few years ago, the island of the Black City was discovered by the viking Harald Olvirson. Blew off course while sailing to attack Hibernia, Olvirson's ships discovered the frozen, mist occluded island of the Black City and made landfall. After spending part of the summer season exploring the ruins, they returned to the Northlands laden with golden triangles, gems, and strange crystals; many slaves and thralls were bought with the booty. Although Harald initially thought he discovered a way to Jotunheim, they would later rename the island Thule.
In the following year, Olvirson returned to Thule with a larger fleet, bringing wood and supplies to build an encampment near the harbor; the vikings began exploring deeper into the city, and also discovered the catacombs beneath the windswept plaza. More explorers came the following year, pledging spoils to Harald in return for harboring near his trade camp. Many notable raiding parties have gone completely missing, their ships left abandoned along the rocky beaches of Thule; the band of Sjolf the Fox never returned from a trek to map the extent of the city walls, and Borgar Trollbeater's thegns never returned from a deep excursion beneath the city. Still, each year, some young Northmen continue to choose delving into the Black City over raiding the mainland. There is glory and a name to be won fighting the exotic monsters of the ruins, and the chance to trade the strange artifacts and treasure of the city for thralls and goods in the southern cities, ensuring prosperity in the long winter at home.
Some of the tablets liberated from the city have circulated through trade to the decadent cities of the south, such as Karkhedon on the inner sea; there, Prince Dion will pay handsomely for more tablets of the Black City, for it is said the seers of his court can use magic itself to decipher the ancient glyphs; they are rediscovering ancient spells of the very highest levels that have been lost to time. Its only a matter of time before the Sea People themselves outfit a fleet to land on faraway Thule, sending their own seers and sorcerers to plumb the depths.
As the concept of the Black City continues to percolate, I've begun conceptualizing the inhabitants and potential treasures to be found there.
When the civilization of the Greys crumbled in a wave of violence, desperate acts were performed throughout the city, including lighting the Great Beacon; a signal was transmitted to the cold wastes of space, in the hope that other colony ships of the Grey race might hear the distress signal and reach Earth in time to help. None have come, but other predators and star demons have followed the signal down to earth, and some lurk even now in the ruins of the city.
Atavistic and primitive humanoids, cut off since the ice age, have continued to inhabit the remote areas of Thule, and some hunt the Black City. Undead remnants of past explorers, soul blasted by dark sorceries, linger in the ruins; one can even encounter skeletal legionnaires from the time of Titus Plotius Nasica.
The fabricated servitors of the Greys, still following their last programs and instructions, perform endless duties in the dark corridors and byways.
And of course, the blasphemous avatar of the dark goddess still lurks in the depths where it was summoned, spawning aberrations and monsters that crawl through the benighted tunnels seeking prey.
As for treasures, the Greys used triangles of gold and silver throughout the city, and the Northmen have been claiming these items as treasure, to be melted into hack silver or forged into coins. They've discovered strange matrixes sheathed with crystals and gems that they loot for the precious stones.
There are also enchanted items of the Hyperboreans, from the forces of Acheron, and from the Northmen themselves, lost in the ruins. And finally, there are the magical implements left behind by the Greys, and the numerous metal tablets displaying their curious, pre-human glyph language.