Plot
In 1941 Romania, following the commencement of Operation Barbarossa, a motorized Gebirgsjäger unit of the Wehrmacht, under the command of Captain Klaus Woermann, arrives at an uninhabited citadel known as 'the Keep', with the aim of taking control of the Dinu Pass in the Carpathian Mountains. Two soldiers, privates Lutz and Anton, attempt to loot a metallic icon within the Keep, but accidentally unleash a spectral entity who kills them. The being, known as Radu Molasar, kills five more soldiers in the following days and begins to take corporeal form.
Woermann's report of the deaths and request for backup is answered by the arrival of a detachment of SS Einsatzkommandos, under the command of sadistic SD Sturmbannführer Erich Kaempffer. Kaempffer claims the deaths are a result of Soviet partisan activity in the village overlooking the Keep. He executes three civilians as collective punishment and takes another five as hostages, despite Woermann's protests.
The Germans find a mysterious message emblazoned on a wall of the citadel. The local village priest, Father Mihail Fonescu, suggests they retrieve a native of the village, ailing Jewish historian Theodore Cuza, from a concentration camp, to decipher it. Upon arrival, Cuza translates the message and says it's written in Old Slavonic, using the Glagolitic alphabet. He and his daughter, Eva, are given quarters inside the Keep.
Soon after, two Einsatzkommandos attempt to rape Eva. Molasar saves her by obliterating the soldiers, brings her to Cuza and, by touching him, cures his debilitating scleroderma. He, then, demands Cuza remove a talisman, which he claims is his, from the Keep. Molasar states this will allow his escape from the Keep's confines and affirms he will, then, destroy the Nazis as he did Eva's assaulters. Cuza, twice indebted to the entity and relishing the emergence of an anti-Nazi force, agrees.
Having remotely sensed Molasar's presence, a man named Glaeken Trismegestus travels from Axis-occupied Greece to the village, carrying a large case. He seduces Eva and incurs Cuza's ire. Questioned by Eva about his origins and identity, Glaeken says his mission is and always has been to destroy the evil within the Keep and that, when that evil is gone, he will be, too. Learning of the plan to remove the talisman, he tells Cuza the talisman is his, not Molasar's, and that its power is all that is preventing the evil within the Keep from being unleashed upon the World.
The malign influence of Molasar begins to affect the villagers, driving them mad. Kaempffer sends a group of soldiers to arrest Glaeken. When he resists, the soldiers shoot and force him into a ravine. Woermann furiously denounces Kaempffer for his sadistic crimes, claiming that the monster hunting the Nazis is a reflection of their evil. Their conversation is suddenly interrupted by the sound of horrible screams and machine-gun fire from the Keep's inner courtyard. Kaempffer shoots and kills Woermann, then goes to the silent courtyard. He finds all Germans slaughtered and all the military vehicles parked inside disabled.
Molasar appears and kills the terrified Kaempffer, while Cuza retrieves the talisman to remove it from the Keep. After acquiring the talisman, Cuza starts for the outside while Glaeken, having survived the fall, climbs the ravine and goes to the inn, grabbing a staff from his travel case.
Eva enters the Keep and attempts to stop her father. Cuza resists and strikes her down, but, when Molasar commands him to kill her, he stops fighting and starts questioning the entity, instead. As punishment, Molasar returns Cuza to his diseased state. Glaeken arrives, retrieves the talisman, combines it with the staff, forming a weapon, and confronts Molasar. The weapon emits beams that interact with the Keep's icons to disable Molasar, turning him back into mist and banishing him back into the innermost recesses of the Keep. Glaeken is transformed in a storm of light and seals the aperture that freed the entity, containing him within, once more. The villagers, freed from Molasar's maddening influence, escort Eva and Cuza away.
