Why is it that when Merry stabbed WK the bonds holding his "flesh" together were broken, but when Frodo stabbed him it didn't do anything? They were both swords of Arnor.
I think you will find all Frodo stabbed was the Nazgul Lord's (the Witch-King of Angmar's) cloak, for as Aragorn said, in 'Flight to the Ford' of FotR,
'...all blades perish that pierce that dreadful King.'
Did anyone catch that in 'A Knife in the Dark' of FotR, "...it seemed to him that it [Frodo's sword] flickered red, as if it were a firebrand"? I hadn't noticed that before. I think Narsil/Anduril flickered red too, but I don't know about Merry's sword, and then again Frodo's may have, only because he was wearing the ring at the time. Oh, and Narsil wasn't a Sword of Westernesse, because it was forged by Telchar of Nogrod in the FA. The highlighted colors are mine.
(Frodo's replacement sword, "Sting" was not a blade of Westernesse. those were made by the Dúnedain Númenórians for use against Angmar; Sting was even older, having been made by the Elves of Beleriand prior to the founding of Angmar and turned it blue in the presence of Orcs.)