Mortdale by Wendy Heiss

A witch.A vampire.And a five-hundred-year-old pact between them.Every summer solstice, a witch must marry a vampire. And in return, a vampire must die as a sacrifice to keep an ancient curse from killing the witches. A pact that was meant to keep peace between Meridian and Mortdale.When word is sent that the witches of Meridian seem to keep dying soon after they are wed to a vampire, Willow leaves her position as an occult pathologist to marry Lord Logan Duncan and investigate the deaths of her coven sisters.Working right under the King of Mortdale as his second-in-command, Logan is forced to keep a close eye on his new wife and figure out what mission she was assigned in Mortdale while also struggling to face a plague of madness that is forcing his species near extinction.