Dexter in the Dark by Jeff Lindsay

Is evil alive...? Dr Jerry Halpern is trying to find out, studying for his PhD on the subject. Dexter Morgan, meanwhile, has a few wicked things of his own to contend with - not least, planning his wedding to Rita to complete his nice-guy disguise. But when a student of Halpern's is found burnt, molested and headless - seemingly sacrificed to an ancient god - and Dex is brought in as forensic analyst to help investigate, he realises he could be dealing with someone a whole lot more sinister than he is. Soon it seems the dark passenger in Dexter's head has gone into hiding. And when something creeps out your friendly neighbourhood serial killer, you know it's serious..