Wizards and Detectives

I quite enjoy these type of urban fantasy novels.  I've read Dresden Files (and seen the TV show), Rivers of London.  I suppose they must have had some influence on this dream but I'm not entirely sure where it came from, anyway, here's the idea:

The detective in question specialises in dealing with cases that involve magic, wizards etc.  The one thing about him is that he can't use magic, which isn't exactly out of the ordinary.  What makes him different is that magic also has no effect on him: spells don't work on him, magical items become ordinary when he uses them e.g. a flaming sword becomes a normal sword if he handles it, basically if it is anyway magical it won't do a thing to him.  


He takes up a case to protect a young pregnant woman from a couple of sorcerers who want to kill her.  Having defeated them, he finds out that they were trying to kill the unborn child, that it isn't human and now poses a direct threat to him.  It feeds on magical energy and, with the danger of the sorcerers out of the way, sees the detective, who is distinctly un-magical, as a danger that needs eliminating.


I think there is quite a bit to work with here.  There is the origins of the detective to sort out: what made him this way? When did he find out about his ability?  What is the down side to the ability?  Where did the sorcerers and the pregnant woman come from?

Powerful  wizards are likely to be fairly arrogant, particularly faced when faced with someone who, at first glance, doesn't pose any threat to them at all.  They are likely to rely on their magic to do things that might be more easily accomplished in a more normal way e.g. rather than use a gun to kill the detective, they will use a spell.  The detectives best weapon is the fact that the people he is after don't know that their powers can't hurt him.