User blog comment:Merlaa/Why "The Diamond of the Day"?/@comment-2.223.93.6-20121221202034/@comment-2.223.93.6-20121221205912

Yeah, it's called "Merlin" by Edwin Muir. Here's the link to the poem:

http://www.poemhunter.com/poem/merlin/

It doesn't actually mention Nimueh in it, but in some of the legends she in the Lady of the Lake and it is her that traps Merlin.

Since Freya is the Lady of the Lake (and she's defiantly good) and Merlin literally obliviated Nimueh in series 1, I doubt either of them will be making an appearance, let alone trapping Merlin in any caves. My best bet is that it's Morgana who traps him.