Descendant

An enemy far from human . . .
Californian James Falcon's compelling Romanian mother told him so many folk stories that by the time he reaches college in 1943, he is something of an expert on the strigoi, the legendary, undying vampires who infested the most isolated forests of Wallachia.
Mostly as a joke, he writes a term paper on the strigoi. But the joke turns serious when US counterintelligence approach him to recruit his expertise.
James hunts down strigoi murderers in war-ravaged Europe, Nazi assassins hired to run down run down the French and Belgian resistance in exchange for Transylvanian independence, although the principal one, the terrible Dorin Duca, continues to elude him. In the Cold War, he must fight once more, as Duca goes on the rampage, spreading his strigoi infection all across London, England.