According to Slavic folklore, a Rusalka is a water spirit or water nymph. They are the souls of young women or girls who died an unnatural or violent death. If the girl was murdered in or close to a lake, she would became a Rusalka and inhabit that particular lake. Rusalkas appear as beautiful young women who try to lure men into the water, where they will drown them. Rusalki can also be found on nights when there is a new moon; dancing on meadows or open places in the woods. With their shrill laughter they are capable of killing humans. A Rusalka's fate can be undone by avenging her death.
Whenever people would bathe in the lake they put fern in their hair so that the rusalka would not pull them under and drown them.