Tales of trolls and forest creatures where common when I was a kid and my family lived in Norway. There are different stories and versions of what a Hulder is.
Hulder Maidens are the beautiful daughters of the trolls that seduce men and lure them back into the forest. The way to know a women is a Hulder is that she will have a cow's tail.
Those who follow the Huldra are lost forever in the mountains. They might be killed, they might be eaten, or they could be under the spell of the Hulder and lured into the troll world deep inside the mountains. Humans captured by the trolls forget the light of the sun and what it is like to be free. The only light they crave is the glow of the gold and riches hoarded by the trolls.
It was said if a man was able to make a Hulder fall in love with him and have a Christian marriage she would become good. At their wedding the sound of the church bells would cause her tail to fall off and she would become human.