excerpt from the article "Norwegian Fairytale Creatures"  via Life in Norway

Nøkken

Where: Lakes and rivers

The nøkk (or nykk) is a water spirit that lives in lakes and rivers. It is a shapeshifter that often takes the form of a white horse or beautiful young man in order to draw unsuspecting victims to a watery grave. Sometimes it also uses music to lure people to it, which often leads to it being confused with another water spirit, the fossegrim. Young unbaptised children and pregnant women are particularly at risk, but that doesn't necessarily mean it's safe to go for an evening stroll around the lake if you don't fit into these two groups, as the nøkk is particularly dangerous after sundown.

Even catching a glimpse of the nøkk or hearing its cry is risky, as it is often a warning that someone is going to drown.

Sceptics may try and argue that the nøkk is merely a way to keep children from playing near deep water. Arguably, sceptics have kept the nøkk well fed for many years.

While the nøkk could be in any lake, Theodor Kittelsen‘s inspiration for his paintings of the creature was Telemark's Tårntjernet lake, which is the biggest lake in Jomfruland National Park. We therefore ask any aspiring fairy-tale travellers to take particular care when walking in this area.

How to escape

If, despite our warnings, you still find yourself caught by the nøkk, there is one thing you can do.

It may be tempting to take a deep breath before you hit the water, but instead you need to use the last of your air to yell the nøkk's name. In the folktale “Nøkken som hest” (The Nøkk as a Horse) (ml4095) from Hedmark, three boys are playing by a river when a white horse emerges from the water and lies on the bank next to them. Immediately, the two eldest boys clamber onto its back. Of course, the youngest one wants to join too, so he calls to his brothers:

“Je skulde ha nykke meg oppaa je eu!” (“I want to get up too!”)

Without meaning to, the boy had said the nøkk's name, and upon hearing it, the nøkk immediately canters into the water, taking the two eldest boys with it but allowing the third to escape.

Norway is known for its range of dialects, and we have not confirmed whether the nøkk accepts all pronunciation variations of its name.

We would therefore recommend to non-Norwegian speaking travellers who are interested in visiting the Norwegian lakes to learn how “nøkk” is pronounced in that municipality – or at least have a string of possible pronunciations prepared, just in case.