Nazgûl Theme

[piosenniel]

I'm beginning to wonder whether the Lord of the Black Rider was actually and finally destroyed.

Yes, Merry struck a mortal blow as did Eowyn (imo), but at the end of the passage in the ROTK, Tolkien writes:

. . . and 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

Does this leave open the possibility for him in a future age to regain his strength and come back?