Outside

The Outside, or Netherworld, is a place not of our reality. It’s first mentioned at the Duel in Death Masks.

Description
It is where Outsiders are locked away from our reality beyond the Outer Gates.

The White Council's Seventh Law of Magic forbids the contacting of Outsiders.

Death Masks
In Death Masks, the Archive produces mordite for the test of wills in the duel between Paolo Ortega and Harry Dresden. They describe the mordite as not of this world, or this galaxy or this universe—not of our reality. According to Harry Dresden, it's from Outside. Being near it draws the life from a person, touching it will kill.

Dead Beat
In Dead Beat, the Red Court vampires pursed the White Council carrying many wounded into Faerie and called up both demons and Outsiders from the Netherworld, the Outside. Only a mortal can call the Outsiders, indicating the existance of a traitor.

Cold Days
In Cold Days, the Outside is compared to World War One, with troops of Outsiders and the Winter Court fighting and moving around the landscape constantly. Physically the Outside is described as a land of dust, mud, and loose shale. It is covered in hillocks and steep gullies. The only plants that grow there appear to be able to to poke, scratch, or sting. The land itself is lit with a sourceless, ambient light, and according to Harry Dresden the sky "was as black as Cat Sith's conscience, without a single star or speck of light to be seen—and it was an overwhelming sky, enormous, like in the open, rolling lands of Montana and Wyoming."