Since "The Unseen" is your basic B movie, the only reason to see it is that it stars Barbara Bach (Ringo Starr's wife), Stephen Furst (Flounder in "Animal House") and Sydney Lassick (Cheswith in "One Flew Over the Cuckoo's Nest" and also the principal in "Carrie"). No, seriously. That's the only reason to check it out: The Beatles, John Belushi and Stephen King are linked by a story of three women staying in a house inhabited by a carnivorous inbred! If only the founders of Solvang had predicted this! Sorry if I haven't done the greatest job describing the movie. It's just that a cast like this easily eclipses the movie, in my view. Granted, there are a few of the requirements for horror flicks, namely the bathtub scene. Otherwise, nothing significant here.
PS: Barbara Bach had previously starred in a similar movie with a similarly jaw-dropping cast. That movie was the Italian horror flick "L'isola delli uomini pesce" (called "Screamers" in the US), in which she co-starred with Mel Ferrer, aka Audrey Hepburn's ex. Yes, it's true. Ringo Starr's soon-to-be wife and Audrey Hepburn's ex co-starred in a movie about half-human, half-fish creatures.