The king of the Kingdom of Fireflies is a sickly man. He has never seen the sun and it has harmed his health more than any of his subjects. It is all he can do to reach his throne room, and he must survey his kingdom from his castle windows.
The kingdom is small, built into the sides of the cavern containing the underground lake Bluelight. Everyone lives in carved out spaces in the walls, lit by glowing blue-green lichen. Some are alcoves with patches of dried lichen too sleep on. Some are as fine as a palace on the surface, or so their owners boast, despite no one having seen the surface in living memory.
Open spaces, side passages and islands in the lake, are reserved for agriculture. The people of the Kingdom of Fireflies grow grubs, planting them upright in the gravel with only their shiny black heads showing. The grubs are fed all manner of organic waste, slowly but surely growing fat. A farmer must judge, without digging up the grub, how close it is to pupating and maturing into an firefly. The closer to pupation, the fatter the grub, but the fireflies are inedible. A portion of each crop is allowed to mature, the fireflies swarming above the fields pulsing frantic mating signals, and under this light the people hold their harvest festivals, gorging themselves on grubs until the fireflies fall, then splitting them open and collecting their eggs for the next planting. The king looks for clusters of wandering lights, so that he can know his people will be fed.
Mermen live in lake Bluelight. They are not like the mermaids of the surface the King has read about. Cave mermen are pale, with semi-translucent skin that has a shiny, slimy look. They no longer have eyes, although they still have eye sockets. Instead of legs they have tails like eels, long and sinewy. They trade silverfish scales and steelcrab shells for tools, which they struggle tomake themselves. The king sees them sometimes, from his bedroom window, silhoutted by glowing algae.
The king's castle is carved out of a rocky promentory jutting out above the lake. It was created to closly imitate the childhood home of his grandfather, who grew up in a castle on the surface. It contains many artifacts of the surface, although few of them have escaped rust or rot. Visitors marvel at the workings of metal their ancestors had wrought. They themselves only know how to craft leather, chitin, stone, and bone.
Someday they will reclaim the surface from the evil that banished them, but the king knows he will not live to see that day.