Ever since I was little my mom told me to never pick flowers near lakes, rivers or small ponds, otherwise Nøkken could come and take me away and drown me.
There is also a way to know if Nøkken is awake and hunting, if there is mist on the lake/pond you can be sure nøkken is there. watchout for glowing eyes if want to pick his flowers.
Even if you are able to run away, a white horse might come after you. Who is this white horse? It's Nøkken himself.