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 the unclean force or нечистая сила (nechistaya sila). 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 die in peace if her death is avenged. In most versions, the rusalka is an unquiet dead being. 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 (срок, srok) as a spirit. Another theory is that rusalki are the female spirits of the unclean dead; this includes suicides, unbaptized babies, and those who die without last rites. (Under this theory male unclean dead were said to become vodianoi)
Rusalki can also come from unbaptized children, often those who were born out of wedlock and drowned by their mothers for that reason. Baby rusalki supposedly wander the forest begging to be baptized so that they can have peace. They are not necessarily innocent, however, and can attack a human foolish enough to approach them.
While her primary dwelling place was the body of water in which she died, the rusalka could come out of the water at night, climb a tree, and sit there singing songs, sit on a dock and comb her hair, or join other rusalki in circle dances (хороводы, khorovody) in the field. Though in some versions of the myth, the eyes shine like green fire, others describe them as extremely pale, with no visible pupils, such as in the famous Ivan Bilibin drawing. Her hair is sometimes depicted as green, and often perpetually wet. According to some legends, should the rusalka's hair dry out, she will die.
Rusalki like to seduce men. They can do so by enticing men with their singing and then drowning them. Men seduced by the rusalka could die in her arms, and in some versions her laugh could also cause death (compare with the Irish bansee). She corresponds to the Scandinavian and German Nix.
In Polish mythology, rusalki dwell in the waters from fall to spring; in some traditions, they reside in the waters from summer to fall. In other tales, they become Sky women when they return from the waters. They are called "Queen of Fairies", and it is said that only witches dare to swim with rusalki. Rusalki brought moisture to field and forest.
Another Polish tradition considers rusalki tree spirits which could be found on nights when there was a new moon, dancing in open places in the forests of Poland.
In Ukraine 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 beautiful men, they would fascinate them with songs & dancing, mesmirise them, then lead the person away to the river floor, to live with them.