Book 2 of the Sign of Seven Trilogy. Book #1 is Blood Brothers. Book #3 is The Pagan Stone.
For Fox, Caleb, Gage and the other residents of Hawkins Hollow, the number seven portends doom -- ever since, as boys, they freed a demon trapped for centuries when their blood spilled upon the Pagan Stone. For seven days every seven years, madness descends on this small town. But three men bound by blood and three women bound by ancestry to a demon have pledged their souls - and their hearts - to stop it.