Aldo nan le Selenya
The ruined city exists, simultaneously, in both the Shadow Realm and Dunia, making it exceptionally dangerous. While the impact of shadow sickness is very limited compared to being in the shadow realm proper, it also means that all manner of shadebeast roam the ruins as well, largely unaffected by Dunia's light.
The city of Selenya must have been gigantic, as the ruins stretch for unfathomable areas in all directions. Traversing the ruins take more than a day, even at pressured pace. That's assuming you find a straight path as well, seeing as the ruins sometimes have rubble, destroyed buildings or collapsed ground stopping progress. On top of that, there's of course the horrible shadow demons roaming the ruins.
At several locations in the ruins, there are passages into the underground, large stairs, dug straight into the substrate and covered with a very smooth stone. These stairs lead down into constructed catacombs, with what appears to be something like ship's docks, made of the same smooth stone. But there's no water down here, just metal rods, evenly spaced, stretching for untold distances in the dark tunnels.
The shadowscar encompassing the city is teeming with shadebeasts. If it wasn't for the presence of floating overlords, it would be possible to traverse the ruins by stealth and avoid the roaming bands of drones and warriors. But the overlords provide their brethren with unnatural perception and will easily spot a group of people moving through the ruins.
Winged and majestic, the Ithonair once ruled all of Celenia. But their progress and greed lead to the destruction of their civilization.