After the Witch-king is stabbed by Merry and Éowyn Tolkien writes..
a cry went up into the shuddering air, and faded to a shrill wailing, passing with the wind, a voice bodiless and thin that died, and was swallowed up, and was never heard again in that age of this world.
What does he mean here? If the Witch-king is mortal and dead then he will have left the circles of the world wouldn't he? Or was Tolkien intending to have him somehow resurrected?