Shadowfell

Mirroring the Mortal Plane, no-one knows how it came to be. Together with the Feywild, it is one of two planes connected to the Mortal Plane through environmental portals. Environmental portals come into existence almost by random, but they all have something in common: they are spots where the Shadowfell/Feywild is almost exactly the same as the Mortal Plane. Most travellers don't notice this however, and soon they turn up in a world they've never seen before...

The Eternal Shadow
While certainly a place of despair, something more fear-inspiring is happening in the Shadowfell: those who stay there for too long, will forever feel the Shadow looking over them...