Jon says, “In some respects quite an ordinary folk-song story, but the imagery is very powerful here. I first heard this sung by Jackie McShee on the John Renbourn Group’s Maid in Bedlam CD, which is great by the way.”
I’ll add that I’ve come across some lyric sets for this that seem to end happily. This Mudcat thread at least goes some way there, but there are even some verses where our newly arrived hero saves the day. Still I agree that this is indeed powerful stuff, a shudder went down my spine.