The Keep is a horror novel by F. Paul Wilson. It is also the first volume in a series of six novels known as The Adversary Cycle. It appeared on the New York Times Bestsellers List and has been adapted for film and as a limited-series of comics.
German soldiers and SS einsatzcommandos alike are being slowly killed off in a mysterious castle keep high in the Carpathian Mountains in April 1941 in WWII Romania. A Jewish History Professor living in Bucharest, Theodore Cuza, and his 31-year-old caregiver daughter Magda, are collected and delivered to this 500-year-old, gloomy citadel in a desperate effort by SS Sturmbannfuhrer Eric Kaempffer to find out what it is that is murdering the men and, if possible, to stop the gruesome carnage by any means necessary.
The professor is useful first in translating a mysterious message that has been written in blood on a wall of the keep in a forgotten dialect of Old Romanian or Old Slavonic. His other more questionable purpose is in finding a religious or supernatural method of defeating the unknown evil power or entity that is wreaking havoc there.
The dark entity sees a certain usefulness in this Professor Cuza and ultimately procures his services through a three-fold indebtedness. First, the demon who deceptively introduces himself as "Viscount Radu Molasar" rescues Magda from sexual assault; Second, Molasar cures the professor of his debilitating scleroderma; Third, Molasar promises to rid the world of Naziism once the newly-healed, able-bodied old man has helped him escape from the keep by removing a certain talisman from it and hiding it somewhere in the surrounding territory. What Cuza fails to understand is that this being is not a mere vampire at all as he pretends but actually something much worse. Molasar, whose actual name is "Rasalom", is an ancient sorcerer from the semi-mythical "First Age" of humans on this planet who surrendered himself to the Powers of Darkness and whose only reason for being is to feed off of human misery and suffering wherever and whenever he is able to find or instigate it.
And so now enters the reluctant champion of the ancient Forces of Light, an immortal man calling himself "Glenn" who is the last surviving member of a group known in pre-historical times as The Glaeken. It was he who built the keep as a prison for "Molasar" out of the reluctance to kill him outright for fear of forfeiting his own life as well. The two beings are mystically linked in a way that binds their destinies together. Rasalom or "Molasar", curiously, harbors no such hesitancy in his attempts to terminate Glenn's physical existence. Furthermore, The two ancient beings are grossly mismatched in their supernatural abilities and it is only the power-sword/Talisman combination which makes Glenn a worthy opponent for Rasalom as the latter has four abilities which the former does not. These 4 powers include non-corporeality, levitation, telekinesis or mind-over-matter, and the ability to draw all warmth and light out of an area and totally disorient the victim.
Magda and Glenn meet and develop a relationship which quickly becomes romantic as Professor Cuza, acting as a stooge for "Molasar", manipulates the German SS into arresting Glenn and bringing him into the keep where he would be vulnerable to Rasalom's ever-increasing power. The scheme fails only because Magda rescues Glenn from dying by bringing him his healing power-sword after he is riddled with bullets and plummets onto the rocks below the bridge leading into the fortress.
Professor Cuza, meanwhile, is excitedly carrying the Talisman through the lower levels of Rasalom's 500-year prison and upwards toward the surface where he plans to re-bury it according to "Molasar's" instructions.
Magda leaves her lover Glenn to recuperate while bodily intervening with her misguided father to keep him from crossing the perimeter of the fortress area with a Talisman that actually belongs to Glenn rather than "Molasar". Removal of the Talisman from Rasalom's prison would de-activate the mystical power that holds him there and he would then be free to roam the wide world at will.
Magda's intervention gives Glenn just enough time to arrive on scene and, joining the Talisman to his power-sword, he is able to drive Rasalom backwards into the depths of the keep. Rasalom then uses his telekinetic abilities to launch an overwhelming assault against Glenn but the latter finally prevails when Rasalom rashly launches himself bodily at his age-old enemy and is apparently slain and reduced to ashes by a single sure stroke from Glenn's power-sword. Glenn plummets far, far down onto the craggy rocks below as Rasalom's body decays into nothingness, and it seems as though not even the power-sword and Talisman together will be potent enough to save him this time.
Glenn eventually awakens in the cellar below and makes three quick discoveries: First, that the Talisman has changed from silver and gold to iron; Second, that he can see his reflection in a nearby pool of water; Third, that the full extent of his injuries amounts to no more than a simple broken leg. He has apparently annihilated Rasalom and survived, albeit as an ordinary mortal man rather than as a near-indestructible immortal being. But there is a bonus.
Magda has again stubbornly refused to write him off and has found him there in the vast, dark cellar of the now-demolished fortress. The man called Glenn now intends to live out the remainder of his limited lifetime with a woman who is both the first to lavish her love upon him in an untold span of years as well as the last one he will ever again love in this lifetime.