The Witch-king and the Barrows of Cardolan

Question

I was reading an answer to a question and in the answer it was mentioned that:

[The Witch-king] had been shaken by the fire of Gandalf, and began to perceive that the mission on which Sauron had sent him was one of great peril to himself both by the way, and on his return to his Master (if unsuccessful); and he had been doing ill, so far achieving nothing save rousing the power of the Wise and directing them to the Ring. But above all the timid and terrified Bearer had resisted him, had dared to strike at him with an enchanted sword made by his enemies long ago for his destruction. Narrowly it had missed him. How he had come by it — save in the Barrows of Cardolan. Then he was in some way mightier than the B[arrow]-wight; and he called on Elbereth, a name of terror to the Nazgûl. He was then in league with the High Elves of the Havens.

From Tolkien's The Hunt for the Ring essay, reproduced in The Lord of the Rings: A Readers's Companion

I wanted to know the story behind the bold lines. I do not have access to the books and the LOTR Wiki was of no help in this regard.

Thank You.

Answer 1

The "missing him" part is just a reference to Frodo's failed attempt to stab the Witch-King at Weathertop:

At that moment Frodo threw himself forward on the ground, and he heard himself crying aloud: O Elbereth! Gilthoniel! At the same time he struck at the feet of his enemy.

...

'Look!' [Strider] cried; and stooping he lifted from the ground a black cloak that had lain there hidden by the darkness. A foot above the lower hem there was a slash. ' This was the stroke of Frodo's sword,' he said. 'The only hurt that it did to his enemy, I fear; for it is unharmed, but all blades perish that pierce that dreadful King.'

The Barrows of Cardolan refers to the barrows in which Frodo and the hobbits encounter a barrow-wight, before being saved by Tom Bombadil.

Tom stooped, removed his hat, and came into the dark chamber, singing:

Get out, you old Wight! Vanish in the sunlight!

Shrivel like the cold mist, like the winds go wailing,

Out into the barren lands far beyond the mountains!

Come never here again! Leave your barrow empty!

Lost and forgotten be, darker than the darkness,

Where gates stand for ever shut, till the world is mended.

At these words there was a cry and part of the inner end of the chamber fell in with a crash. Then there was a long trailing shriek, fading away into an unguessable distance; and after that silence.

...

For each of the hobbits [Tom] chose a dagger, long, leaf-shaped, and keen, of marvellous workmanship, damasked with serpent-forms in red and gold. ... Then he told them that these blades were forged many long years ago by Men of Westernesse: they were foes of the Dark Lord, but they were overcome by the evil king of Carn Dûm in the Land of Angmar.

The Encyclopaedia of Arda provides a good history (and map) of Cardolan, including the Barrow-Downs where the remnants of the Cardolan residents resided until ousted by the Witch-King.