Third Dark Lord, or Melkor's Return?

[Galin]

I think the Last Battle (and so on) survived in a sense... as in a "Ragnarokian" end of sorts, but that the Second Prophecy of Mandos did not survive.

"The myth that appears at the end of the Silmarillion is of Numenorean origin(19); it is clearly made by Men, though Men acquainted with Elvish tradition."

JRRT

(19) "... in so far as the reference is to any actual written text, this is the conclusion of QS (...), the Prophecy of Mandos."

CJRT

Morgoth's Ring, Author's note 7, Notes on the Commentary, Athrabeth Finrod Ah Andreth. Note 19, Christopher Tolkien

It makes sense to me that Mandos no longer delivers "the end" as prophecy, but Men have myths about it, and one in which Turin returns from the Doom of Men at the ending of the World (and possibly Beren Camlost does something -- see the cursory corrections to the end of LQS in the War of the Jewels).

That said, in a late text (see note 17, The Problem of Ros, The Peoples of Middle-Earth) a prophecy is given to Andreth, the Wise woman (still not Mandos of course), that Turin will return from the dead before his final departure, and slay Ancalagon the Black... although according to Christopher Tolkien, this refers to the Last Battle at the end of the Elder Days, not a last battle at the end of days... and here it is noted, before Turin's final departure.

Given this late muddle, while I think there was going to be some mannish end-type myth here, I'm not sure about all the details. Yet if a Mannish myth, why not have Turin (a man of course) come back and deal with Morgoth...

... I mean, such a saying is not going to be easy to disprove anyway