Tiernokarst
The True Kingdom of the depths, survivors of an apocalypse centuries past and all their brave descendants, surviving in the wreckage. Then growing, changing, devouring all that already lived below, twisting them to their purposes. The last bastion of humanity. Or so they believe.
Established by the first Queen and her followers, the first to accept and understand Amalgamation. Her devotees gained new strength, magic, skills and longevity. Other bands of survivors could not resist the draw of power for long, or if they could they could, they could not hold out once the crystal set in.
The Kingdom in earnest began in the earliest caverns of the Greens. Survivors turning decaying debris and waste into fertilizer,and using new magic, even burning out their souls to fuel it. Mutating what seeds they could scavenge from the collapse, and burning even more life to imitate solar radiation. Desperate echoes of the world above to make it through that first decade or so of starving.
Some independent communities still survive, still denying the Queen and her blessing. Largely in more remote caverns and spread across the wider reaches of the lakes, but they keep to themselves, and live very carefully. If they do not, well, all in Tiernokarst will certainly appreciate that the markets have an influx of cheap, new limbs.