A long time ago, people used the Underdark. Suddenly, however, historical records of the region went... well, dark. This was several hundred years ago. Local legends and stories are told of terrors that slumber deep below the surface of Aeonara, and not all are entirely fiction.
The Underdark is a vast expanse of caverns that lies beneath the surface of Aeonara and is rumored to extend down to the planet's core. Certain caves are supposedly connected to it, but there are no truly viable sources from people who have been there.
Some elves fled to the Underdark during the Calamity, a catastrophic event that nobody quite knows the full story of (save for perhaps the gods). This event is the reason that Snow Elves are the only large population of elves on the surface and why there are no more chromatic dragons, or so scholars speculate. These elves are supposedly still down there. Only rumors exist of these elves.
Great monsters, such as umberhulks, formorians, and abberations dwell in the Underdark. It's a dangerous place to be. Local legends tell of the creatures rising from nearby caves and stealing children in the night.