Based on European (Germanic) Mythology
Undines are of the darkest depths of water. Their majik comes with the feeling of that pressure and their water is never clear. Their water is cold to the point it is almost ice.
They are vengeful creatures thought to have no soul unless they married a human. If their spouse is unfaithful the human will die while the undine will be unable come out of their vengeful state, projecting their hatred on anyone who resembles their spouse in any loose manner.