Jim Butcher Dresden Files Down Town (2015) #3

With the help of Bob and a rather twisted spirit entity, Harry has tracked a killer to Chicago's deadliest supernatural neighborhood: the labyrinthine network of subterranean tunnels known as Undertown! It's a place even powerful mages fear to tread, but Harry has little choice if he wants to prevent any further bloodshed. At least he'll have Molly with him�if, of course, she survives her own encounter with their enemy's remains.