User blog comment:Merlaa/Why "The Diamond of the Day"?/@comment-2.223.93.6-20121221202034

"The Diamond of the Day" is a line from a poem about Merlin being trapped in a cave by Nimueh, I think. In the trailer, it looks like Merlin gets trapped in a cave like in the poem (though probably not by Nimueh) - it's probably a reference to that.