Third Dark Lord, or Melkor's Return?

[Mithadan]

It is unclear whether the Prophecy of Mandos survived Tolkien's revisions to the Silmarillion. If I recall, later versions did not include it. He may have rejected it in favor of his view that Middle Earth merged into the "Primary World" (the Age(s) of Man) following the end of the Third Age.

His conception of the Prophecy was that at some point the Valar would become weary of the world and relax their vigilance, permitting Morgoth to re-enter Arda. The Final Battle would follow after which the world would be remade. Some versions had Turin slaying Morgoth if I recall.

Assuming that the Prophecy remains part of the mythos, Morgoth would return in his own body. Some earlier writings had Morgoth being executed following his capture after the fall of Angband. Tolkien rejected this idea for a few reasons. First, he felt that it was not in the nature of the Valar to execute him. Second, Morgoth's death was inconsistent with his later conception of the Valar as perpetual and not able to be "slain." They can be weakened to the point where they cannot construct a physical body, but their spirit would remain. The final iteration was that Morgoth was physically bound with Angainor and expelled into the void. So he had a body for what it's worth.