The Shadowfell

The Shadowfell is a scarcely traveled area of the world. A series of tunnels that span through most of Aderia, they are mostly abandoned at this point due to tunnel collapses. Remnants of the Age of Dragons where the survivors would hide from the skies, they are now relegated to collapsed tunnels with the occasional hole at the bottom of the mine.

Geography

Comprised of tunnels with the occasional settlement or underground lake, the shadowfell's geography has access to no light.

Fauna & Flora

The wildlife in this area are mostly scavengers and foragers that live of the bioluminescent mushrooms in the area. This does include some larger threats such as ankhegs but a properly prepared traveler should be able to get by with little issue. 
Some pathways are covered in a thick carnivorous moss. This moss is highly thermareactive and will be adequately subdued with anything about as hot as a torch.

History

The Shadowfell originated as a system of natural tunnels deep underground. During the Age of Dragons, the tunnels were expanded to have a place safe from dragonkind. 
After the Banishing, the tunnels were left abandoned for the most part. The vast majority of them have collapsed in at this point, although some settlements still exist to this day.