There would have to be some very good reason why a terrifying tale of the supernatural, written by no less than Robert Louis Stevenson and surpassing in power and impact even Dr Jekyll and Mr Hyde itself, is hardly known.
The reason is simplicity itself - the story is written (all but its first two paragraphs) in broad Lallans Scots, the near-vanished dialect of south-western Scotland from Ayrshire to the Solway. For all that, it's still English, not Gaelic, and any English-speaking reader who can cope with Rabbie Burns ought to be able to get the hang of Thrawn Janet. Stevenson begins in standard English, switching to Lallans when recounting the story as it might have been told in the early 18th century by an elderly parishioner of the clachan of Balweary who remembered (who could forget?) the events of the terrible night of 17 August 1712. This story is not fantasy, like, say, Mervyn Peake's Boy in Darkness. Christianity can see off any fantasy-writer when it comes to sheer panic-inducing horror, and here we have a full-blooded narrative of diabolic possession as that would have been recognised in an isolated and superstitious rural Presbyterian community. There are no pagan elements as in the stories of M R James. Nor does Thrawn Janet deal in hints, ambiguity and innuendo like Henry James's Turn of the Screw. This story is not just 'in your face', it grips you by the throat - the sickly sweltering heat of that awful night, the huge black stranger, the ghastly mangled speech of Janet herself, the lonely ordeal of the minister in his lightless manse. There are only 16 or 17 pages of it, and if you can't get your ear around the dialect you may sleep better for not having been put through it, but you will have missed a rather unique experience all the same.
I have no idea at all of the origins of the story. For all I know it may have come straight out of Stevenson's own imagination, but I'm inclined to suspect he was working with some kind of legend. If there ever was some Janet M'Clour I wonder what she ever did to deserve what befell her. Having borne a child out of wedlock would hardly by itself have brought this fate upon her even by the ideas of that time and that community, but rumour being what it is she would easily have come under suspicion of being in league with the devil. As Stevenson relays the tale, it ends with the minister living out the rest of his life obsessed with his experiences like the Ancient Mariner. The devil is exorcised, but there is no theme of redemption. German-speakers may know Moerike's poem The Fire-rider, the theme of which is along the same lines. At least the poet, if nobody else, finds it in his heart to pray for the soul of the possessed fire-rider, and perhaps the rest of us can do the same for poor Janet.