The Devil's Rock (2011)

On 5 June 1944, a unit of New Zealand commandos are sent to the Channel Islands on sabotage and distraction raids, to draw the German military's attention away from Operation Overlord, planned landings in Normandy.

Captain Ben Grogan and Sergeant Joe Tane land by canoe on a fortified beach on Forau Island. Heading inland, they hear distant screaming and gunfire. Infiltrating a German fortification, they hear what sounds like a man being tortured. As they begin placing explosives on artillery guns, a German soldier runs out of a tunnel pleading for help. Grogan stabs the soldier, killing him. The two investigate the bunker separately and discover the mutilated bodies of German soldiers. As Grogan ventures in further, Tane discovers a book of black magic and, distracted by its contents, is killed by an unseen assailant. Grogan, unharmed, later discovers Tane's body but is knocked unconscious.

Grogan wakes and is tortured by Nazi Colonel Meyer, who wants to know his mission. During the interrogation, Grogan hears a woman screaming from another room. He eventually escapes and chases Meyer into the tunnels, shooting and injuring him. Upon tracking the woman's screams to a room covered in occult symbols, Grogan discovers that the woman is his dead wife, Helena. Meyer enters the room and shoots Grogan in the leg, then shoots Helena in the head. Meyer explains to Grogan that the woman is a demon, summoned from a book of black magic found on the island. As proof, Meyer offers her a human leg; she changes into her true demon form as she devours it.

After Grogan removes a bullet from his abdomen, Meyer passes out. Grogan searches him and discovers a page torn from the book of black magic in a small pouch worn as a necklace. Grogan keeps the page after replacing it with another from the book. Soon after, Meyer recovers and explains the demon is a shapeshifter and a weapon the Germans plan to use against the Allies. As it cannot cross moving water, the demon is confined to the island. However, Meyer now realises the demon poses too great a threat to the world. Meyer offers to give the book to Grogan if he will help him escape from Germany. Meyer persuades Grogan to help him perform a ritual to dispel the demon back to Hell. Meyer, believing he is protected by the incantation sheet from the book, betrays Grogan at the end of the ritual.

As Meyer reveals his true intent to use the demon for the Nazis, Grogan overpowers Meyer and throws him to the demon, who brutally kills him. When she tries to convince Grogan to take her with him as Helena, he tells her that she could never replace the real Helena, and he chains her up again. Unable to complete the ritual alone, Grogan takes the book and leaves the demon behind, to prey on any Germans that come to investigate. He explains to the demon that he intends to come back when the war is over to finish the ritual and to banish her forever. He leaves the key to the chain within reach as he leaves the demon in the bunker. He steps onto the beach, buries a photo of Helena he kept with him, and watches as D-Day begins.

In the post credits scene, a German soldier enters the tower, finding dismembered body parts and empty chains. The demon appears behind him, having taken another human form. It starts talking to him, its voice slowly changing to its demonic one.