Huldra

Huldras are powerful, nearly-human fey. They all take the form of beautiful human women, with red hair (of varying shades) being nearly universal, and two distinctly fey features: a fox tail, and a hole in their back that reveals their insides to resemble a hollow tree.
While their personalities closely resemble the range of human personalities, as part of their fey nature Huldras share a singular focus on personal autonomy. Any hint that an acquaintance intends to force something upon them against their will, from taking liberties with their person to simply dropping by unannounced and expecting to be fed, can cause them to erupt in a violent rage, completely out of proportion to any such perceived offense (from a mortal's perspective, at least). Even those whom they’re fond of can be on the receiving end of a deadly curse if they ignore the warning signs of a Huldra’s anger at being taken for granted for more than a few moments - apologizing profusely and immediately leaving her presence is generally the only way to avoid catastrophe if a Huldra’s fey nature is triggered this way.
Provided that they are not avenging such a slight, though, Huldras are generally quite pleasant to be around, and they often visit human or near-human settlements to try and find companionship. These visits usually end in heartbreak for the community and fury for the Huldra, though, as the average human settlement always has at least one person who is guaranteed to provoke the fey’s anger.
There are four rules for dealing with Huldra’s: Be polite, pretend her tail and the hole in her back don’t exist, NEVER imply that you intend to take liberties with her person, her possessions, her services, or her friendship, and always be willing to leave her presence (immediately, and without any sign of disappointment or hurt feelings) if she asks you to.