According to most legends, the Rusalka is a fish woman that lives in the bottom of rivers. She is similar to mermaids or sirens, lururing handsome men down to the bottom of the river with songs and seductive dancing.
Most people believe that this dark mermaid is a kind of "unclean force". Because of it being a female apparition, it is believed she either committed suicide because she was jilted by her lover, or that she wasn't married when she became pregnant.