The Outsiders are.. dangerous to say the least. They're the one specific group of beings that the White Council has a rule about which is simply "NO CONTACT! No summoning them, no talking to them. Fight or run only." Or at least that's the gist of it. Cold Days covers a lot of details on the Outsiders, but there's still more about them that's sketchy and in the dark. 

Basically in the Nevernever,, deep in, there's the Outer Gates which are a doorway to another realm entirely it seems. There has been battle raging around the gates for who knows how long. It has been Mab's job to fight much of this battle and keep the Outsiders from getting in. He Who Walks Behind is an Outsider. They're creatures of great power, very dangerous, and most human magic rolls off of them with little to no effect. Sidhe magic is a bit more effective, but not much more. It comes down to melee combat a lot. Rashid the Gatekeeper (Senior Council member) is involved and has been helping in the situation. Mab has committed millions of Winter fae to the fight, and most of the trouble Harry's had in the past has been a small fraction. 

Anyway, Outsiders are baaaad juju. If you had it in mind, what city might you be interested in?