In Russian folklore, a Rusalka was the spirit of a girl who died a violent, or unnatural death near to a lake.
Rusalkas inhabit the lakes near which they died, and appear as beautiful young women. They attempt to lure men into the water with their beauty, so as to drown them.
On nights when there is a new moon, Rusalkas can be found dancing in open meadows, or in open places in the woods. If a person were to come upon them, he or she would be killed by the Rusalkas' shrill laughter.
A Rusalka's fate can be undone by avenging her death.
Also, a book by C.J. Cherryh. The book deals with a young boy, who is learning to be a wizard, and his older brother. His brother falls in love with a Rusalka, who is the daughter of the young boy's teacher. Lucky for the older brother, this Rusalka likes him, and does not attempt to drown him. Throughout the course of the book, the young boy learns what it means to be a wizard, and his brother attempts to restore the Rusalka to life.
In this book, wizarding is a rather unpleasant business. Magic is accomplished simply by wishing hard enough, and so the boy must take great pains to avoid wishing for the wrong things, and is always afraid that his idle desires will cause bad things to happen.