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The Thalkast is a network of underground pathways almost incomprehensibly complex for surface dwellers to grasp. Numerous underground civilizations have existed in the world, many predating the Sporespawn. It was only under the Overmind that some semblance of order was attempted to be instilled on the hundreds of miles of tunnels, causeways, bridges, and arches connecting the three main Hive Halls.
The construction and excavation of this magnitude took decades of labor with all the resources the Overmind could muster. This made its collapse into darkness all the more painful after The Sundering of the Sporespawn into the three main factions at the turn of the Heroic Era.
Pitched battles between the Sporespawn settlements have rendered many of the roads and bridges impassible from rubble. There are passages leading into hundreds of smaller caves that descend deep, deep into the bedrock of Dema, where thousand legged Cave Horrors and goopy Snottite colonies prey on those looking to traverse the dark. While efforts at recolonizing dozens of the smaller halls began in earnest over the Uprising Era, the total area safe to travel is a small fraction of what was once settled and secured.
This makes expeditions by Ikhish researchers or bands of surface adventurers especially profitable when they pull up ancient Eldertech or rusted energy weaponry left behind by past societies. They fetch a high price between Sporespawn, who look for some edge in the seemingly endless wars between the Everborn, the Freespawn, the Awakened, and the encroaching mechanical Brass Automata.
The natural features of the Thalkast are just as grand and treacherous as the constructed. In extensive chambers, the ceilings of the caverns extend hundreds of feet high, entire ecosystems thriving in overgrown tangles of glowing fungi cropping up next to underground lakes and rivers. Explorers tell of entire catacombs taking up chambers the size of towns, their walls and ceilings a lattice made entirely of skulls from unknown sentients. Trace remnants remain of some massive living organism known as the Rifteater that once bore tunnels a hundred feet high. Whether it still lives is a question not to be whispered around the fading fires of travelers lost in the caves.
Of note is a Sporespawn legend which tells of a set of stairs carved by primordial demons that descends into the underworld, where a city of the dead called Pandaemonium sits abandoned for thousands of years. It is from here that some believe the mechanical Brass Automata were created in the God Workshop, a black-stoned pyramid so tall it touches the stalactites a mile above its foundation. Luck be it to the traveler who could ever approach its borders, with the creatures that have escaped shattered glass enclosures full of green liquid skulking the crypts and vaults. Even these monsters shy away from the Forbidden Cells, chambers beneath the Mountains of the Moon, which are said to contain prisons for mega entities even the Overseers could not kill.
The possibilities for adventure are just as vibrant below ground as above. The Exodus Era Hegemony's morale took decades to recover from the disappearance of two entire legions sent into the Thalkast to aid their Sporespawn allies against an unknown threat, with only one insane survivor returning to friendly territory begging for the passage to be blasted shut behind her. The network of tunnels presumably extends far beyond the original borders the Overmind laid claim to in the northeast Nightlands. Perhaps it extends unmapped far across the other five regions of Dema, with lost cultures and treasure hoards of your own design waiting to be raided for treasure.
Buried in the dreams of fallen cultures lay secrets only the most intrepid heroes can bring to the light. What waits for your characters in the bedrock abyss?
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