[Nerwen]

"Bewildering" is an anagram indicator.

And no, the theme is not "immortal creatures".

I mentioned "at the Pelennor" as being a hint towards the overall answer/theme. Now that the clue has been solved, I will explain that it's very important that it was that Gothmog, and not Gothmog, Lord of Balrogs. You just have to think why that might be.

The phrasing of the "were-worms" clue is also a hint.