I recently read something about a classic ballad (probably in the Child index). Unfortunately, all I remember is that it had the same story as the German ballad "Der wilde Wassermann" http://ingeb.org/Lieder/EsFreitE.html. That is, a river-monster forcibly weds a maiden; she wishes to leave, but cannot desert her children (especially as there's an odd number of them).

Also, I notice that Faun performs "Der wilde Wassermann" to the tune of the Staines Morris Dance. Is that their own idea or does it have more history?