Perhaps the most fearful characteristic of water nixies is their allure.
These magical creatures appear as beautiful women, but they trick fathers into giving up their sons or outright kidnap and enslave children who fall into wells. My early medieval characters grew up hearing tales like “The Water Nixie” and “The Nixie of the Mill-Pond”. These beings have such a hold on the imagination, that they, not the protagonists, are in the titles, and what might have frightened listeners most is that the villains, although defeated, are still alive and able to wreak more havoc on mortals.
See my post on Unusual Historicals for more about water sprites that terrified my characters.