Dark Creature Seduces Men

by Zev
(New York, USA)

I saw a movie in the 80's on TV where a creature comes out of a dark ooze or swamp, then transforms into a woman who seduces men.

She gets them to cheat on their girlfriends and then kills/feeds on them. The woman morphs into different forms for different people. At first she has a claw and then it turns into a hand.

In the end, one of the creature's victims fights off his desire, and he and his girlfriend defeat it.

What is this movie? Does anyone know?