Meriadoc, I'm afraid that the "cloud-shape" wasn't the Witch-King, but another Nazgul. Then the WK died, it was said that his voice was not heard again in that Age of the world - and Frodo and Sam heard the Nazgul scream. The scream didn't hold power over them because the Nazgul was in great hurry and distress.
Yet there is evidence for the theory that the WK wasn't actually killed in the battle at Pelennor (I believe in this theory myself). Tolkien once wrote (footnote to Letter 246) that the WK had been "reduced to impotence" - not "killed" or "slain".
Peregrine, no one but Iluvatar can bring back a Man who has died. There are some ways to delay the death, or to bind the Man's life to some object (like the Nazgul), but there can be no way to bring back a Man after the death.
There is an exception, of course... in the tale of Beren and Luthien Beren died but he was brought back from the halls of Mandos, before he had left the World completely. It would appear that Mandos (or perhaps Manwe) has the power to let a Man go from the Halls of Waiting in Mandos - but I don't think that anyone else, ever, was granted this than Beren, and no one most definitely can be forced to leave.
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