Hello there! I hope this is okay to ask as I don’t know if this is exactly in your wheelhouse, but I’ve been trying to do some research for a while now on hardanger fiddles and their place in folklore/folk practices and beliefs, and I wondered if you’d have any information you could point me to about it or books to recommend? I’ve come across some bits of information and leads but I don’t know how reliable they are. I know this isn’t really heathenry-related and something more recent, so please feel free to ignore! Thank you though for any help you can provide!
I mostly know Icelandic folklore, and there isn't a whole lot about fiddles there. I don't know if there's anything there about hardangers in particular, and fiddles in general weren't very common in Iceland; a few were imported in the 19th century and it's said that a handful of Icelanders were able to make their own, and it supposedly had a reputation for attracting huldufólk.
I'm not sure what languages you're able to read, or your tolerance for retyping things into Google translate. If you're sticking with English, the book Scandinavian Folk Belief and Legend, edited by Reimund Kvideland and Henning K. Sehmsdorf, has a bunch of stories about fiddles and water spirits (nøkk/näck, fossegrim), and then a little bit about fiddles and the devil, all with citations. It's not hard to find a PDF of this book.
54.14 "They Danced themselves to Death":
Finally a minister heard about the dancing on the mountain, and when he commanded the fiddler to stop, the man had to obey. But the folks who had followed the minister saw that it was Old Nick himself who sat there and played. He used his ass as a fiddle and worked it with the bow.
If you're able to read Scandinavian, the book -og fela ho lét. Norsk spelemannstradisjon by Arne Bjørndal and Brynjulf Alver is cited frequently by the above. It's available to borrow one hour at a time on archive.org. Studier i svensk folklore by Tobias Norlind is also available online, and the second volume of the Atlas of Swedish Folk Culture is available to download here.
The Swedish magical tradition includes spells to cause someone's fiddle strings to break; some of these can be found in Svartkonstrböcker ed/trans by Thomas K. Johnson.