The Sacrifice Game

At Blackvale Academy, an all-girls boarding school, two students, Clara and Samantha, have stayed behind for Christmas with their teacher Rose and her boyfriend Jimmy. A group of murderers, Jude, Maisie, Grant, and Doug, have been terrorizing the nearby towns with a series of ritualistic murders based on a demon summoning ritual Maisie discovered while attending Blackvale. A chance encounter with a police officer forces the killers to take refuge in Blackvale, where they quickly capture and restrain Rose and the others, killing Jimmy in the process. They decide to finish the ritual at Blackwood by slaughtering Rose, only to grow frustrated when the promised demon doesn't show itself.
Maisie goes to the basement with Jude to investigate the spellbook where she obtained the spell, in the process discovering that she had only obtained part of the ritual and that the final sacrifice must be someone unpure. Doug goes after them, leaving Clara and Samantha alone with Grant. Jude wanders off after acting strangely. As Maisie reads the spell, she realizes that the ritual is intended to free a demon, Clara. Clara is then shown using her powers on Grant and compells him to free Samantha, whom she instructs to leave her behind and flee for safety. The demon then uses her powers to mentally overtake Grant, using him to capture and render Jude, Maisie, and Doug unconscious. Samantha manages to flee to a security station and call the police, but chooses to return in order to save Clara.
Maisie awakens to discover that Clara has imprisoned them without their clothes in order to complete the ritual. Clara reveals that she was imprisoned within the school walls centuries earlier after the townspeople changed their minds while summoning her. Clara forces them to make various personal sacrifices, culminating in the deaths of Doug and Jude. Maisie manages to break Clara's control over Grant, who kills himself. Samantha returns and discovers that Clara is the demon, further breaking Clara's control and freeing Maisie who begins fleeing in only her underwear. Upset, Clara demands to know why Samantha returned; she responds that she couldn't leave a friend behind. The two argue, as Clara refuses to believe that anyone could care about her. She asks that Samantha free her by killing Maisie as she escapes into the snow. Samantha complies and the film ends with her and Clara leaving to travel the world together.