[Mithadan]

Legalos does a fine job of addressing the source materials of this legend. As he comments, the concept of Dagor Dagorath is very unsettled. Some argue that Tolkien rejected the idea ultimately due to his linking of his mythos to the real world.

Others suggest that he modified it and intended the version quoted from Peoples of Middle Earth to be an amendment of the battle at the end of the first age in which Morgoth was overthrown. Advocates of this position suggest that the reference to the great dragon means Ancalagon the Black. Some also point out that the latest conceptions of the Battle of Wrath suggest that Morgoth was not exiled to the void but was instead executed so that there could be no final battle.

The reappearence of Turin goes back to earlier conceptions of the final battle and is not consonant with other parts of the legendarium which provide that Men depart from the world after they die. The concept of Morgoth being slain can also be cast in doubt as the killing of an Ainu would likely fall afoul of Eru's "rules" discussed in Osanwe Kenta.