Rusalka
In Slavic mythology, a Rusalka was a female ghost, water nymph, succubus or mermaid-like demon that dwelled in a lake. She was considered a being of evil force. The ghostly version is the soul of a young woman who had died in or near a lake (many of these rusalki had been murdered by lovers) and came to haunt that lake; this undead Rusalka is not invariably malevolent, and will be allowed to rest in peace if her death is avenged. In most versions, the Rusalka is an unquiet spirit. 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 out of wedlock, must live out their designated time on earth as a spirit. Another theory is that rusalki are the female spirits of the unclean dead; this includes suicides, unbaptised babies, and those who die without last rites. (Under this theory male unclean dead were said to become vodianoi).
Read more about this topic: Supernatural Beings In Slavic Folklore