The Winchesters find that they have been beaten to the punch by another demon hunter. By way of compensation, though, he directs them to Mission Hill, Georgia. Strange events have happened there at the site of the Civil War battle in the book's opening passage.
The story takes some time to get going after Sam and Dean arrive in Mission Hill. The town sheriff has a gruesome mystery to solve that is connected to a Civil War reenactment happening outside the town. She isn't impressed when two oddly named FBI agents turn up and start asking lots of questions. When "Townes" and "Van Zandt" meet the angel Castiel (introduced in Supernatural's fourth season) they find out that the significance of the battle being recreated at Mission Hill is more than historical.
Castiel is in Mission Hill looking for an infamous figure from Christian writings who can lead him to an object that is behind the paranormal events in the town. He is not the only one. Demons start possessing the locals and it becomes apparent about a third of the way through the book that things are going to get bloody.
As the body count rises the story picks up pace and becomes a compelling read. Perhaps because I haven't seen much of the series, I did find myself getting lost in the lore underpinning events. The deceits in the plot also left me confused by exactly how that famous historical figure fit into the story. Once I decided to roll with these uncertainties, though, I was able to enjoy the action-packed last two-thirds of the story without getting too distracted.
The Supernatural television series has successfully trodden a fine line between horrifying its audience and maintaining mainstream appeal. The Unholy Cause does the same thing, although it might be too action oriented for avid fans of psychological horror. The early parts of the book have their creepy moments, but later events lean more towards spectacle (albeit with a supernatural twist) than scares. There is not much here that will frighten or disgust anyone other than small children (although those black-eyed demons did give me the heebie-geebies). Even so, it was exciting enough to keep me coming back.