Beneath the surface of Vosca there is another world. A world of labyrinths, and great halls, and endless caverns snaking deep into the earth. An endless, dark matrix of silent tunnels. Some of them are volcanic in nature, while others were crafted by some unknown force eons ago. This subterranean world is collectively called "The Hollow." Some say that at its lowest depths there are great chasms as deep as the sky, with endless columns leading down into pits of eternal darkness, where dark things creep and whisper.
Many ancient things dwell in this lightless place. The Goblins dwell in the upper levels, and Dwarves often pick into them to use them for travel and to find new sources of ores and precious stones to mine. The Burdinadin often tap into them for their own purposes.
But few peoples, even the dark-sighted Goblins, dare to venture into the lower levels. They say that there are to be found things that should never have been, horrid relics of bygone eons left resting beside slumbering, ageless abominations.
A cold wind endlessly shrieks through The Hollow. It comes and goes in pulses, chilling the stone and shifting the dust around. Nobody is quite sure where this wind comes from. Some speculate that it originates in the great chasms at the bottom of the known network, but that just raises more questions.
Despite the terrifying nature of the place there are people determined to conquer it, just as they have conquered the surface. The Dacian Crosswalkers and The Gladiolus Society have both made efforts to seize control of the upper levels of the labyrinth in the vicinity of their prized trade route. Momentary success can be found, but The Hollow always takes itself back, eventually.
There is a war of escalation between these surface factions and the deep, and it shows no sign of slowing. The forces of the dark places always seem to push back harder than they are pushed against. Humans or Goblin tribes pour into the tunnels, and Horrors emerge, or plagues, or worse.
Most people, even the Dwarves, are wise enough simply to leave The Hollow alone, but there are certain areas, such as the Karthakasel Cross Way, that are too important to neglect. The Karthacks have gone as far as to employ sorcerers to try and drive back the darkness, but like all surface efforts against the depths that has only made things worse. The Dacians have tried everything from storming the depths of The Hollow with hundreds of armed men to flooding the tunnels with poison gas and then setting it on fire, with absolutely calamitous results.
Thankfully, the effects of this escalation are localized. The rest of the world does not suffer much for the antics of the Dacians and the Karthacks.