The Keep by F. Paul Wilson
"Something is murdering my men."
Thus reads the message received from a German commander stationed in a small castle high in the remote Transylvanian Alps. And when an elite Nazi SS extermination squad is dispatched to solve the problem, the men find a something that's both powerful and terrifying. Invisible and silent, the enemy selects one victim per night, leaving the bloodless and mutilated corpses behind to terrify its future victims. Panicked, the Nazis bring in a local expert on folklore--who just happens to be Jewish--to shed some light on the mysterious happenings. And unbeknownst to anyone, there is another visitor on his way--a man who awoke from a nightmare and immediately set out to meet his destiny.
The battle has begun: On one side, the ultimate evil created by man, and on the other... the unthinkable, unstoppable, unknowable terror that man has inevitably awakened.