It still feels a bit too early in Advent for proper Christmas carols, though I will undoubtedly change my mind about that very shortly! But this one kind of fits in with the Advent 1 ‘Wachet Auf / Sleepers Awake’ sort of carols.
This is a 20th century arrangement of a 15th century verse or carol, and the lyrics are very strange to a modern ear. In particular, keep an ear out for Mary’s title in Verse 3. Very metal!