Is there any description on how Morgoth will destroy the Sun and the Moon in the Dagor Dagorath?

Question

it is said that Morgoth will destroy the Sun and Moon in the Prophecy of Mandos:

"Thus spoke Mandos in prophecy, when the Valar sat in judgement in Valinor and the rumour of his word was whispered among all the Elves of the West. When the world is old and the Powers grow weary, then Morgoth, seeing that the guard sleepeth, shall come back through the Door of the Night out of the Timeless Void; and he shall blacken the Sun and the Moon."

is there anything in the History of Middle Earth that would indicate a plan of doing this or is it just a factoid of a Judgement Day predestined?

Answer 1

Primary Source

"Thus spoke Mandos in prophecy, when the Valar sat in judgement in Valinor and the rumour of his word was whispered among all the Elves of the West. When the world is old and the Powers grow weary, then Morgoth, seeing that the guard sleepeth, shall come back through the Door of the Night out of the Timeless Void; and he shall blacken the Sun and the Moon. But Eärendil shall descend upon him as a white and searing 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 Morgoth, and on his right hand shall be Eönwë, and on his left Túrin Turambar, son of Húrin, coming from the halls of Mandos; and the black sword of Túrin shall deal unto Morgoth his death and final end; and so shall the Children of Húrin and all fallen Men be avenged.

The best I could find regarding the mechanism:

According to the prophecy, Morgoth will discover how to break the Door of Night, and will blacken the Sun and the Moon. For the love of these, Eärendil will return from the sky and shall meet Tulkas, Manwë, Eönwë (with the old version of his name Fionwe) and Túrin Turambar on the plains of Valinor.

There the forces of the Valar shall fight against Melkor and The Dark Powers. Tulkas will wrestle with Morgoth, but it will be by the hand of Túrin that finally death and destruction will be dealt to Melkor.

Break the door of Night and Blacken the Sun... But a battle will ensue meaning I think it's the effect of this combat (terrible war) that will blot out the Sun and Moon. Smoke, fire etc. will have that effect both at night and day.

Of note is that the prophesy itself is brought into question:

It must be mentioned here that "Dagor Dagorath", the name properly said, was not used by Tolkien in The Lost Road or in The Shaping of Middle-earth. All the occurrences of the term in these books were in the editorial notes of his son. The canonical mentions of the name are only in Unfinished Tales. Christopher Tolkien included the name in The Shaping of Middle-earth because the first mention of the Second Prophecy of Mandos was placed there but Tolkien did not mention the existence of a Prophecy in the text published in Unfinished Tales. The manner with which the name appears in Unfinished Tales suggests that there are some kind of "foretelling" of The End but does not state that it comes from one of Mandos' prophecies.^source

Answer 2

I think this is more poetic than anything else, a metaphorical vessel to describe the impact of a one true embodiment of evil returning to Middle-earth.

Interesting to note that most of good and evil in Tolkien's universe is represented by light and dark. The light of the two trees of Valinor, the Star of Eärendil that blinded Shelob, the fact Orcs avoid sunlight, and Trolls downright have to avoid it for their own survival. The light of the Silmarils, which is actually from the same source as the two trees. The lamps (which are the Sun and Moon). All vessels for the representation of good and evil, and most of Morgoth's work is the undoing of the light.

Answer 3

The second prophecy of Mandos was written during the phase where Tolkien had the sun and moon as bright fruit from the two trees carried around Arda in vessels by the maiar.