Lai du Cor

AVI found a great song by a group I'd not heard of before. In the comments I mentioned the source for the magical horn that reveals false lovers: it's a pretty neat Anglo-Norman Arthurian poem called Lai du Cor from the 12th century. There's a summary of the tale and its lessons here. 

It's not one of the better-known Arthurian stories in our time, but it was very popular in its day.