Oh yay you Sachael, that's fantastic to remember that. I hadn't thought about them.
The Rusalki are water Demons. They can look a little like mermaids, and are not always very nice.
Apparently they haunted the waterways where they lost their lives, and if they saw a young man while they were dancing there in the moonlight they would charm him with song and then lead him away to the river bed to live with them in the murky depths.
If you should happen upon a Rusalka dancing in the moonlight the advice given is to move away as fast as you can.
I'm so glad they told me that, because after all I may never have thought of that all on my own!

I remember reading about them (renamed as the Riselka, but still recognisable) in the novel Tigana by Guy Gavriel Kay.
By the way this is a fantastic book, that I really loved when I read it. I think now I really must dig it out and read it again over the summer.

This is his website for anyone who is interested
Bright Weavings the official website of Guy Gavriel Kay