Wolf in the Fold — Simon R. Green

Every family has its black sheep. But the wealthy MacNeils of Haven have a terrorist in their bloodline--with an emphasis on blood. Hawk and Fisher are quick to learn that even high society has its share of lowlifes, and they plan to go undercover to expose the criminal.
But the skeletons they find in the closet might be their own.

They're two tough cops in a city of magic and mayhem. Hawk rules the streets by battle-axe. Fisher wields her sword and dagger with unflinching skill. Together, they are the perfect crimebusters ...with a magic touch.