After that, you're on your own, but just that seems to be a gold mine...shrunken heads used to trap the souls of enemies? Check. Severed heads speaking prophecy? Check, make it one verse per head and you've got a serial killer. Mad scientists trying to perform head transplants? Check.
And as a twist for the Dresdenverse: collecting heads of minor talents to bolster your own. Twist it as you see fit ^_^;
That's a sick but REALLY fun twist. I can hear the "There Can Be Only One" jokes already.
I think I'm going to go with the prophesy angle for the actual killer just because the prophecies let me set up a future case for the PC's. Now if only I could find a random prophecy generator that I could set for extra doom with a side of hallucinogens and tentacles.
A red herring that recently occurred to me is a White Court vampire of clan Malvora, nothing like a serial killer to ramp up fear after all. Might have him working the crime beat for the local paper. Sound like something typical PC's would look into?
Well they'll be motivated if they knew one of the decapitated victims, or if someone they knew only just escaped with their lives.
If it's doom and tentacles that you're after, you could have an outsider speaking through the heads? It would sound like prophecy but it's actually a cunning plan to manipulate the serial killer, sending him on missions to create or prevent a particular future when actually it's just opening a gateway for the outsider (plus friends) to pour through. That way the serial killer could be someone they'd never suspect, like a nun (on a secret holy mission to decapitate key minions of the anti-christ or whatever); a family man (protecting his children from the horrid future in the prophecy) or whatever. Fun :)
Well they'll be motivated if they knew one of the decapitated victims, or if someone they knew only just escaped with their lives.