I would like to discuss the nature of the death of men in Tolkien’s Legendarium. There are two questions to be answered here, (1) what happens to men after they die? and (2) why do they die?
Unfortunately, the first question does not have a very complete answer. The elves believe that the fëar of Men go to the Halls of Mandos after dying, but only stay there for a short while. Afterwards, they depart from the circles of the world completely. Tolkien does not explain completely what happens to the fëar of Men, and given his attention to detail we must assume this is left intentionally vague. Finrod states, in the Athrabeth, that Men must have estel, or hope. He explains that this word does not have the same meaning as when men say “hope”, but is closer to our trust. Finrod thinks that Men should trust that Ilúvatar will care for them, and that after death they will not be abandoned. Andreth has trouble seeing this hope and says it is easy for the Eldar to trust because they know what happens to the fëar of the Eldar after death, but Men have no knowledge of what happens to them. The best answer we are given to this question is the fëar of Men leave the circles of the world after death, but this results in a sundering of their fëar and hröar.
The second question is never fully resolved either, but Tolkien wrote a lot more about it. The first answer is that of the Elves and the Valar. They say that Ilúvatar gave to Men a Gift that made them different from the Elves, or any other beings in Arda. The Gift had several aspects. These aspects were that they would always desire something beyond the world, that they could shape their own lives beyond the Music of the Ainur, and that they would only reside in the world for a short time. The first aspect means that they are never satisfied within the world. Ilúvatar says that because of the second aspect of the Gift, Men will help bring the world to fulfillment. The third aspect is the death of Men, which is the most mysterious part of it. The story of Ilúvatar’s Gift to Men comes from the Silmarillion, which is the history of the First Age and earlier, according to the Elves. In the Athrabeth, Finrod confirms this version as the Elvish one when he tells it to Andreth.
The next explanation of why Men die comes from Andreth in her debate with Finrod. She says that when Men first awoke they did not die. She says that the belief that Men have always died is a lie that comes from the Shadow. She also says that the Wise of her people say that dying is not in their nature, but is the fault of Melkor. She refuses to tell Finrod the story of how Melkor caused death to come among them. Finrod reasons that unless the Elves are grievously wrong, Melkor does not have the power to change the fate of an entire people, thus Eru must have changed the fate of Men because of something Men did. In the Tale of Adanel, Andreth does explain how death came to Men. Very simply, Men began to worship Melkor because he provided knowledge to them and Eru punished them with Death. Again, the origin of this story must be considered. This story is told by Adanel and Andreth, who are Edain. The Edain eventually rebuked Melkor and went West to try to escape his rule. Simply put, the Edain are the Men who realized that Melkor was Evil. Therefore, it is likely that another story would be told by the Men who stayed in the East and likely continue to worship Melkor.
So which of these explanations of death is correct? In his debate with Andreth, Finrod says that Men must have been fated to die from the beginning, otherwise they would just be Elves, and then what is the point of two separate kindreds? Andreth says that he is missing the point, that the Wise among Men are not concerned with the Elves, and just say that Men should not naturally die. The case of the half-elven, when the Valar give them the choice to be either Men or Elves, seems to support Finrod’s claim. If the Valar were wrong in offering this choice, Ilúvatar would have surely corrected them. Even though this tale involves Men and Elves, it still comes to us through the Elves, so there is still potential for bias.
Independent evidence should therefore come from stories of Men. The main story of Men is the Akallabêth. This tale was recorded by Men and therefore should be free of Elvish bias. The relevant part of the Akallabêth is the ability of the Númenorean Kings to die at will, at least initially. When the Númenoreans were still free of the Shadow, their Kings willingly gave up the throne and also their lives. When the Númenoreans begin to hang on to their lives until the bitter end, the Shadow begins to come back. As they grow more and more fearful of death, the Shadow grows in Númenor. This alone seems to be good evidence that death is natural for Men, and avoiding it is rejecting the will of Ilúvatar.
Given that the Númenorean’s descent into Shadow is mirrored exactly by their fear and rejection of death, it seems that the Death of Men must in fact be the original will of Ilúvatar. This is the belief held by the Elves and Valar, but they see it as a Gift because they are so long-lived that they eventually begin to envy Men (indeed, in the end, even the Valar themselves will envy the Gift of Men). The Wise among Men disagree with this view, and believe that Death was imposed on them, but the story of Númenor supports the story the Elves take, so it seems most likely it is the correct reason why Men die. It seems that Tolkien never wanted a definitive answer to this question, but reviewing many of his works, even older versions present in the histories, it is difficult not to come down on the side of the Elves.