[Maédhros]

Here is a compilation of The Prophecy:
From The Book of Lost Tales II: Turambar and the Foalókë

Then Úrin departed, but would not touch the gold, and stricken in years he reached Hisilómë and died among Men, but his words living after him bred estrangement between Elves and Men. Yet it is said that when he was dead his shade fared into the woods seeking Mavwin, and long those twain haunted the woods about the fall of Silver Bowl bewailing their children. But the Elves of Kôr have told, and they know, that at last Úrin and Mavwin fared to Mandos, and Nienóri was not there nor Túrin their son. Turambar indeed had followed Nienóri along the black pathways to the doors of Fui, but Fui would not open to them, neither would Vefántur. Yet now the prayers of Úrin and Mavwin came even to Manwë, and the Gods had mercy on their unhappy fate, so that those twain Túrin and Nienóri entered into Fôs'Almir, the bath of flame, even as Urwendi and her maidens had done in ages past before the first rising of the Sun, and so were all their sorrows and stains washed away, and they dwelt as shining Valar among the blessed ones, and now the love of that brother and sister is very fair; but Turambar indeed shall stand beside Fionwë in the Great Wrack, and Melko and his drakes shall curse the sword of Mormakil.'

Then it changed again in the Sketch of the Mythology or 'earliest Silmarillion'.
From The Shaping of Middle-Earth: The Earliest Silmarillion

When the world is much older, and the Gods weary, Morgoth will come back through the Door, and the last battle of all will be fought. Fionwë will fight Morgoth on the plain of Valinor, and the spirit of Túrin shall be beside him; it shall be Túrin who with his black sword will slay Morgoth, and thus the children of Húrin shall be avenged.

Now we go to The Quenta. From The Shaping of Middle-Earth: The Quenta

Thus spake the prophecy of Mandos, which he declared in Valmar at the judgement of the Gods, and the rumour of it was whispered among all the Elves of the West: when the world is old and the Powers grow weary, then Morgoth shall come back through the Door out of the Timeless Night; and he shall destroy the Sun and the Moon, but Eärendel shall come upon him as a white flame and drive him from the airs. Then shall the last battle be gathered on the fields of Valinor. In that day Tulkas shall strive with Melko, and on his right shall stand Fionwë and on his left Túrin Turambar, son of Húrin, Conqueror of Fate,7 and it shall be the black sword of Túrin that deals unto Melko his death and final end; and so shall the children of Húrin and all Men be avenged.
7. Added here in pencil: coming from the halls of Mandos.

We have also a Commentary regarding this part. From The Shaping of Middle-Earth: Commentary on the Quenta

The appearance of Túrin at the end remains profoundly mysterious; and here it is said that the prophecy names him among the Gods, which is clearly to be related to the passage in the old Tale of Turambar (II. 116), where it is said that Túrin and Nienor 'dwelt as shining Valar among the blessed ones', after they had passed through Fôs' Almir, the bath of flame. In changes to the text of Q II it is said that Túrin is named among 'the songs of the Gods', rather than among the Gods, and also that he comes 'from the halls of Mandos' to the final battle; about which I can say no more than that Túrin Turambar, though a mortal Man, did not go, as do the race of Men, to a fate beyond the world.

We have also a reference of Túrin returning in the Annals. From Morgoth’s Ring: The Annals of Aman

On the two star-makings see p. 61, §24. There is here the remarkable statement that Menelmakar (Orion) was 'a sign of Túrin Turambar, who should come into the world, and a foreshowing of the Last Battle that shall be at the end of Days.' This is a reference to the Second Prophecy of Mandos

Yet in the Later Quentas, the Menelmakar, had removed as a sign of Túrin Turambar.
From Morgoth’s Ring: Later Quentas

Here the two star-makings are expressly contrasted, and Varda's names Tintallë 'the Kindler' and Elentári 'Queen of the Stars' differentiated in their bearing. The second star-making is described also in AAm§§35 - 6 (p. 71), but far more briefly, and though the 'gathering together of the ancient stars' to form signs in the heavens is mentioned there also, only the constellations Menelmakar (Orion) and Valakirka are named. That Menelmakar forebodes the Last Battle is said in both sources, but LQ does not name it as a sign of Túrin Turambar.

Now we come to an interesting change in the philosophy of The Silmarillion, when it changes from being made from an elvish perspective to a mannish one.
From Morgoth’s Ring: Myths Transformed

What we have in the Silmarillion etc. are traditions ... handed on by Men in Númenor and later in Middle-earth (Arnor and Gondor); but already far back - from the first association of the Dúnedain and Elf-friends with the Eldar in Beleriand - blended and confused with their own Mannish myths and cosmic ideas.

And then he have this piece from The Peoples of Middle-Earth: The Problem of Ros

17. The language of the Folk of Haleth was not used, for they had perished and would not rise again. Nor would their tongue be heard again, unless the prophecy of Andreth the Wise-woman should prove true, that Túrin in the Last Battle should return from the Dead, and before he left the Circles of the World for ever should challenge the Great Dragon of Morgoth, Ancalagon the Black, and deal him the death-stroke.

So we see the change that have taken place among the place of Túrin in the Second Prophecy of Mandos.
First he was made to come back from the dead and slay Morgoth with his black sword, then that conception changed to slaying Ancalagon the Black. The question that arises is can this prophecy be accounted as true?
From Morgoth’s Ring: The Valaquenta

Here ends The Valaquenta. If it has passed from the high and beautiful to darkness and ruin, that was of old the fate of Arda Marred; and if any change shall come and the Marring be amended, Manwë and Varda may know; but they have not revealed it, and it is not declared in the dooms of Mandos.

If Manwë and Varda may know, but have not revealed what would happen in the end of Arda, and Mandos has not declared it, how can The Second Prophecy of Mandos be true then? We have also this little bit.
From Morgoth’s Ring: Athrabeth Finrod Ah Andreth, Note 7

It is noteworthy that the Elves had no myths or legends dealing with the end of the world. The myth that appears at the end of the Silmarillion is of Númenórean origin;19 it is clearly made by Men, though Men acquainted with Elvish tradition. All Elvish traditions are presented as 'histories', or as accounts of what once was.

So, it seems clear that the Prophecy of Mandos, (at least the part awarded to Andreth), is a revelation given to her, but if the Eldar had no knowledge of the end, how could she know? Was it foreknowledge?
It is interesting to notice how the idea changed in the different stages of the legendarium.