Outside

Outside is a place not of our reality. Mentioned at the Duel in Death Masks.

In the Series
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. Harry says it's from Outside. Being near it draws the life from a person, touching it will kill, period.