Note: This is not a review. Rather it is a vibe-based recommendation for a very specific section of my audience. The Ritual does have issues, but those issues aren't relevant for the intended purpose of this recommendation.
If you are looking for some horrific inspiration for your Dolmenwood game, The Ritual by Adam Nevill might be the folk horror you need.
The story is straightforward enough, four friends hike into the Swedish forests and only one escapes with his life. In between is a goat cult, Nordic black metal bands, and a great deal of atmosphere reminiscent of the Wormskin zines.
Given that this is a book and not something meant for TTRPGs, I don’t recommend pulling specific plot beats from it, instead I recommend stealing some of the setting details, and worldbuilding ideas for a Dolmenwood game.
Perhaps you are in need of some creepy reggle religious rites, it wouldn’t be that much work to adapt something from The Ritual. Same thing if you need inspiration for a Crookhorn base in the deep parts of the forest. I am planning on using it as inspiration when my players find an Atacorn. Or maybe if they meet the Hag.
Again, I don’t recommend pulling specific plot beats from The Ritual. I’m sure it can be done, but the end result is likely to be a lot more linear than Dolmenwood is meant for. GM’s running a sandbox should focus on neat scenarios that allow for player ingenuity, not predetermined plot points.
All that said, while I enjoyed The Ritual, your mileage may vary. For one, this is a very adult piece of media and not one I'd recommend for kids or teenagers. Secondly, while I think the book as a whole is quite good, the first half has some glacial pacing that might frustrate. Lastly, this read definitely embraces horror and not weird. If you lean into the stranger aspects of Dolmenwood, this might not be a good source of inspiration.
If you would like to buy a copy of the ritual to read for yourself, you can do so at the affiliate link below:
The Ritual (Amazon Link)