Here is an entertaining and informative article on huldras, the mysterious and often seductive tailed folk of Scandi forests. The huldra is described as a beautiful woman with a tail that sometimes peeps out from under her peasant attire, but sometimes she's just full-on naked and lures unwary Nordic chaps to what may be their doom. However, as with all such beings there are many and varied ways in which they are said to interact with us mortals.
Some huldra or huldrekarl are inherently deceptive and evil, but many respond to the treatment they receive. If treated kindly, they have been known to use their magic to help humans and solve their problems. If treated unkindly, they can be hateful and vengeful. Much like any other being in the world.
The forest huldras were held to be kind to colliers (wood burners that make charcoal) and watched their charcoal kilns while they rested. The colliers knew that she would wake them if there were any problems. This allowed the to sleep and be rested. In exchange for her help they left provisions for her in a special place.Fans of the Norwegian film Thale will already know that it takes the huldra myth and runs with it in an interesting way. It remains one of the best folk horror movies of recent years, not least because of the way it gives the huldra 'magical' powers that make sense and work well in the context of plot and characterisation. I've recommended Thale before. If you haven't seen it, give it a try! Fans of Nordic noir will appreciate the twists it gives the genre.