Disclaimer: This story is written by group of people and it'll be our first attempt on something like that. We don't own Tolkien's characters (obviously) and don't make any money from it.
Foreword
This story is based on the second prophecy of Mandos as mentioned in The Shaping of Middle-earth
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 Earendel 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 Fionwe and on his left Turin Turambar, son of Hurin, Conqueror of Fate; and it shall be the black sword of Turin that deals unto Melko his death and final end; and so shall the Children of Hurin and all men be avenged.
The Silmarillion as J. R. R. Tolkien originally wrote it ends with a prophecy by Mandos about the Dagor Dagorath, often referred to as "The End". The remaining clue to this prophecy is found at the end of Akallabêth, where "Ar-Pharazôn and his mortal warriors who had set foot on Aman were buried by falling hills, imprisoned in the Caves of the Forgotten until the "Last Battle and Day of Doom".
But that's the legends and this will be our spin on it. We have also taken the liberty of referencing the Book of lost Tales as well as other theories from several discussion boards.