Murder camp

(This book include a hidden demon, a satanic force and campers)
A camp full of teenagers ages 18 are at camp deep in the woods. On the 3rd night, weird disappearances of friends start accruing. One night, a group of teenagers go out to find the people that disappeared but come across the horror of seeing them piled up and ripped apart. The buses are broken and they have no way back home. Unfortunately, the murderer was a campus kid who got possessed by an unknown force. This murderer is only after one person.