In most versions, the rusalka is an unquiet dead being, associated with the "unclean force". According to Zelenin, people who die violently and before their time, such as young women who commit suicide because they have been jilted by their lovers, or unmarried women who are pregnant, must live out their designated time on earth as a spirit.
The ghostly version is the soul of a young woman who had died in or near a river or a lake and came to haunt that waterway. This undead rusalka is not invariably malevolent, and will be allowed to die in peace if her death is avenged.
(In Slavic mythology, a rusalka (plural: rusalki) was a female ghost, water nymph, succubus or mermaid-like demon that dwelled in a waterway.
According to most traditions, the rusalki were fish-women, who lived at the bottom of rivers. In the middle of the night, they would walk out to the bank and dance in meadows. If they saw handsome men, they would fascinate them with songs and dancing, mesmerise them, then lead the person away to the river floor to live with them. The stories about rusalki have parallels with those of Hylas and the Nymphs, the Germanic Nix, the Irish Banshee, the Scottish Bean Nighe and the Romanian Iele. See Slavic fairies for similar creatures.)