excerpt from the article "Norwegian Fairytale Creatures" via Life in Norway
Where: Farms and forests
At first glance, the hulder looks like a very beautiful yet ordinary human woman. At second glance, however, you may notice that she has a long, cow tail – which is when you'll realise that she is far from human. Stories of the hulder are particularly common among dairy farms in the mountains, where they usually wander out of the forest to find a young man to seduce and/or marry. However, there are stories of men trapping an unsuspecting hulder and taking her home to marry (ml5090). In these cases, a hulder will lose her tail as soon as she says her vows and become fully human.
There are also stories of “huldrekall” (male hulder) taking human women, but these are much less common. We do not recommend marrying a hulder during your stay in Norway, purely on the basis of the amount of paperwork that would be involved.
The hulder can also be generous, especially if you do them a favour. In “Jordmorsegna” (Midwife to the Fairies) (ml5070), a huldrekall asks a human woman to act as a midwife for his hulder wife, who is about to give birth. The woman agrees, the hulder baby is born, and the woman is rewarded handsomely.
Their duality is reflected in their musical talents. Like the nøkk, the hulder can use their music as a way to lure people to them. Alternatively, the hulder may teach human people their songs and skills, like the fossegrim.
As with many of the creatures listed here, as long as you're respectful to the hulder, you often have no reason to worry. If you are approached by a hulder asking for help, be sure to do your utmost to fulfil their request and you will probably be well compensated for your trouble. But what if the hulder wants to marry you, and you a) aren't really looking to settle down or b) are otherwise engaged? If this is the case, then “throwing steel” (aka literally throwing a steel knife or shooting a steel bullet) over the hulder will take away their power.
In the story of the “Huldrebryllup på setra” (The Hulder Wedding on the Farm) (ml6005), a young girl is sitting alone when a group of “huldrefolk” (hulder people) suddenly surround her and start preparing her as a bride to an old huldrekall.
She manages to send her dog to alert her boyfriend of the situation, who comes running and “throws steel” over them. The huldrefolk flee and the couple very practically decide that since she's dressed as a bride, they may as well marry.
In short, be nice to the hulder, but always best to have steel on hand. You may also want to make sure that you have a good aim.
Illustrasjon til “Troldskab” (Illustration for “Magic”), Theodor Kittelsen, 1892