Based on loosely 'The Lurking Fear'.
John (Roy Dupuis) and Kathleen (Kristin Lehman) Strauss are a couple attempting to uncover the secret to John's rare blood disease. Along the way, they encounter Dr. Marlowe (Rutger Hauer), who is intrigued by the case. Little do they know that the island which they are about to set foot upon is home to the Van Dam family, mutant-like creatures who have become deformed and bloodthirsty from centuries of inbreeding. Their mutation began with their relative Eva Van Dam, who had an incestuous relationship with her twin brother. Also, they are fully functioning hermaphrodites, capable of reproducing with themselves. Conveniently, they need to survive on (dead or alive) human flesh.
John eventually discovers that he is in fact a Van Dam, who was born normal looking, and was therefore allowed into normal society. His rare blood disease stems from the fact that he needs human flesh and sex with his siblings in order to function properly.
Was released as 'Hemoglobin' in most of Europe. Also known as The Decendant.