In the HoME series, I don't remember exactly which book, Tolkien's notes state that one day, far distant in the future, Morgoth will sneak through the Door, when Earendil's watch is relaxed, and bring devastation into Arda. Earendil will chase him out of the upper sky, and on the fields of Valinor, a last battle shall be fought. Some accounts also state that Turin will deal Morgoth his death-stroke, with his black sword, but others say that Morgoth will be defeated by the Valar. In this conflict, all of Arda will be destroyed, thus enabling Morgoth to be destroyed (since Arda is his "Ring").
The Turin theory is hotly debated, and many say that Tolkien intended to scrap it. I'm sure that there is a thread floating about, perhaps in the Book forum, about the End of Days and Morgoth's Return.
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But Melkor also was there, and he came to the house of Fëanor, and there he slew Finwë King of the Noldor before his doors, and spilled the first blood in the Blessed Realm; for Finwë alone had not fled from the horror of the Dark.
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