This is the final Law of Magic, as written by the White Council.
As I wrote elsethread, "... WoJ has it that each of the White Council's 'Laws of Magic' is going to have a book in which Harry seriously breaks that law..."
And I'm having a REALLY hard time trying to imagine Dresden -- Starborn, Outsider-hating Dresden -- opening the Outer Gates. I guess this happens in the final act of the final book of the final trilogy -- the BAT, Big Apocalyptic Trilogy.
The only way I can envision Harry doing this is if he has a metric crapton of Outsider-Killing magic, opening the Gates and killing EVERYTHING that comes through. But that, in turn, is power to dwarf all mortal power, exceeding even what Darkhallow!Harry would have held; more like the power an archangel wields! Mab herself is merely fighting a holding-action (and an imperfect one at that, since we see the Outsiders keep breaking through)