Sudden Darkness - Erdmann L. R.

It's a slow day when private investigator Bill Radner and his friend John Cleveland meet Alice Johnson. She's concerned over a letter received from her brother--a letter that was cruel and sounded nothing like him. Even though Radner suspects no more than a family argument, he agrees to look into her case.

Turns out Radner was right to work for Alice. Her brother, Adam, is not only a successful trader on the Chicago Exchange but he's also the leader of a satanic cult. Radner enlists the help of Bishop Mark Frazer, and soon, the PI, his pal Cleveland, and a man of the cloth are embroiled in a scandal of devil worship and murder.

Good thing Radner has some ideas. They're going to need to trap Adam before he can kill Alice. The perfect time to do so will be during a ritual sacrifice at Adam's home in Wisconsin on Halloween. It won't be easy, not with Adam's legion of followers, but Radner has backup and a full proof plan to stop this evil mastermind.