The Netherworlds is a world of dreams, or nightmares, however you want to look at it. It's a place we've all been, even if we are unaware of it. 

This is a world where everything is whatever you want it to be, even when you don't know what you want. It was created by the shadows of human hearts and imagination. Spirits created it as a reprieve for when their myths become forgotten, and it was created by the gods as a place for their temples and their halls. 

Here's the catch, though: the Netherworlds was there before they created it. It was there waiting.