Undines are a group of water elementals that are almost universally depicted as having a female form. Mermaids, naiades, nereides, nymphs, and limnads fall within this grouping. It is said they lack a human soul until marry a human and bear him a child. Any man who is unfaithful to his undine bride is destined to die, but the lore that I could find didn’t add any details to the method or immediacy of the death. Since we’re all destined to die, I’m not sure this is a potent enough curse to keep a man faithful if he has desires to go that route.
There is a considerable body of work that leverages undines across the ages, even up to a video game from 1993 that directly uses an undine spirit. (There is a game from 2015 that people think a character is named after undines, but it’s not a direct link.)
My modern usage would be a tale in which a man purposefully pollutes a body of water. To enact their revenge a pair of undine sisters work together. One of them seduces and marries the man before bearing him a child. Then the other sister entices the man to cheat on the first sister. The man then dies a short time later (I’m thinking by drowning), leaving the two sisters to raise the half-human child in a manner that befits their desires for clean water, renewable energy, preservation and protection of the environment, and cleaning up after his now-dead father’s acts.
I could see that story being a three-part novella of sorts. A little horror. A little urban fantasy. A little tragedy. Lots of justice.
“Undine.” Wikipedia, Wikimedia Foundation, 29 Jun. 2020, en.wikipedia.org/wiki/Undine.