According to Dagor Dagorath, which is basically Middle Earth’s version of the Apocalypse, Morgoth will return when the world, along with the Valar and the elves grow weary. Arda will be broken and changed drastically, and war will happen in Valinor. Turin will be the one who kill Morgoth personally. Feanor will break the Silmarils and rekindle the two trees of light again. Beleriand will emerge from the sea again and Arda will be reshaped by the second theme of Illuvatar, which is greater than the first, as men, elves and dwarves will all join the theme. I suppose this could sum up Arda’s and Morgoth’s ultimate end according to Tolkien’s vision.
The tale of Dagor Dagorath has been an exceptionally interesting topic, because it serves like the ultimate ending and beginning which Tolkien has envisioned for everything which exists in Middle Earth. You may try to learn it from Nerd of the Rings in YouTube, I think he makes a detailed explanation over this topic.
