The Lareusian underworld is a vast underground cave network that is almost everywhere if you go deep enough. The complex thousands of tunnels and caverns are impossible to map even for the the Garsosha or Twisted. However major locations have been marked on maps with the surface world's map as a referance point.
The dark underworld is lit by cracks in the rocks that stretch to the top, or more often strange glowing crystals and moss. The main residents are strange creatures or the Garsosha and The Twisted. A rumored creature is the Macroceiri crabs that could have gotten lost.
It's creatures are known for relying on stealth in the darkness, and in a world where they all use such tactics, surface people rarely are prepared to survive below the earth. There are a few creatures who aren't stealthy, such as giant spiders that use webs, or large powerful creatures that use their strength to kill. Almost everything down here has extraordinary eyesight in the darkness.
However there is still a lot of unknown factors about the mysterious underworld.