I really enjoy the way you describe Legolas' ability to hear and sense the inner Songs both of people (Elladan, Elrohir, and how different they are), and also of things: the stones of the City and the song they sing to Legolas of Gondor and its past, and the gorgeous line "Star-song chimed, metallic and distant". Very Elven.
So many convincing little details, like the "sounds of the night" outside the tent.
I was right there with Legolas menaced by the Nazgul, feeling all that black malignity - and the impossibility of keeping out of your mind the thing you know you must not think of...brrr. And then Elrohir like something out of First Age legend confronting it. And the Nazgul in his head too.. the visions it shows Elrohir could be very fandom-cliched, but I think you handle them deftly enough - and we know they're a dark fantasy - to get away with it...
And oh dear, what's going to become of poor Elrohir now the Dark Lord's been in his head?