Really glad you posted this, it's really interesting to hear something so rooted, so untainted by anything modern.
I am one who is really grateful that you posted the translation. Obviously the lyrics are the lyrics, and the translation is the translation, and the one isn't a substitute for the other... But I really wanted to know what it was all about and how the story was set, and being a non-Welsh speaker my other option would be Google translate - which would obviously do a much greater disservice to your lyrics than your own translation does!!... So thank you for that.
Based on the translation, the story is told really well. Lots of great detail that conjures the tension and foreboding. Nice references to the natural world, weather etc.
Back to the song.... The music supports the storytelling really well I think. You're right about sinister - the guitar communicates that really well. Your vocal is strong. The melody isn't the sort of melody that really gives your vocal a chance to show off, but, like the guitar, the melody and vocal do really support the flavour of the storytelling, so I think they work really well in this piece.
Really really great to hear. It's a challenge for us non-Welsh speakers and non-trad musicians....but definitely a rewarding one, I think!
Welcome to the forum, I hope you stick around.