The Witch King's death

[Aiwyn of the Eorlingas]

So passed the sword of the Barrow-Downs, work of Westernesse. But glad would he have been to know its fate who wrought it slowly long ago in the North kingdom when the Dúnedain were young, and cheif among their foes was the dread realm of Angmar and its sorcerer king. No other blade, not though mightier hands had wielded it, would have dealt that foe a wound so bitter, cleaving the undead flesh, breaking the spell that knit his unseen sinews to his will.

The blade that Merry used was most likely made especially for slaying threats from Angmar, the Witch-king's former realm.

breaking the spell that knit his unseen sinews to his will.

But who is HE? That's my question. If 'he' is the Witch-king, then all that means is "his invisible (unseen) tendons were severed, and he could no longer walk (bend them to his will). But if 'he' is Sauron, the situation instantly becomes even more complex. Perhaps, if 'he' is Sauron, Merry's sword severed Sauron's connection with the Witch-king: a connection that could have conceivably provided the Witch-king's 'invulnerability', which gave Éowyn the opportunity to slay him, because it DOES mention flesh in the quote.
The prophecy? I think it could go both ways. Éowyn, while being of the race of Men, was not a man, per se. Merry, being a hobbit, was also, techinacally, not a man.