Here is the summary of The Oath found on Amazon:
Under cover of darkness, something evil is at work in Hyde River, an old mining town deep in the mountains. Its latest victim, nature photographer Cliff Benson, was brutally killed while camping -- and his wife Evelyn has been driven nearly mad by what she saw, but she can't remember what it was. The sheriff thinks a rogue bear killed Cliff. But townspeople whisper -- and Cliff's death is just the latest in a long string of bizarre "accidents." Cliff's brother Steve is determined to find out the truth about what's concealed in the old caverns near Hyde River, a mystery that the local folk legends only hint at.
This review contains some spoilers. The Oath was a masterpiece in the sense of the sin metaphor and spiritual things taking physical form. When I first saw the 550-page book, my heart cowered at the thought of reading through the boring introductions to the characters. These thoughts were unnecessary, as Peretti dragged me in from the first page to the last with turns, twists and plunges. The only thing I found that bothered me was that one or two of the book’s scenes was somewhat inappropriate, even though they didn’t describe anything in detail. 4 out of 5 stars.