orichalcum

It's slightly less obscure than it appears. It's just that it is always performed in a church setting. It is the traditional opening hymn for the King's College service of 9 lessons and carols, and so it's been adopted by many churches as the opening hymn for their carols services, as well. The way it is usually performed is as a processional, with the first verse sung as a boy's solo, and the rest of the choir progressively joining in on subsequent verses. It's very nice. I definitely recommed getting a recording of the King's College choir doing their service.