Gift of Men
But to the Atani I will give a new gift. -- Words of Eru Ilúvatar From Quenta Silmarillion 1, Of the Beginning of Days
Death; the gift of Ilúvatar to his Younger Children, allowing them to go beyond the confines of the Arda. After death, Mortal Men were gathered in the Halls of Mandos, and then departed from the World for a destination unknown even to the Valar. Whereas all other beings in Arda, including the Valar themselves, were bound to the World and its fate, the Gift freed Men from this destiny, allowing them to shape their own lives as they wished.
Those Men with the greatest understanding treated death as the Gift it was originally intended to be, and when their time came gladly gave themselves up to it. We see this, for example, in the earlier Rulers of Númenor, and Aragorn also accepted the Gift at the natural end of his life. For most Men, though, the Gift was tainted by Morgoth, and they came to fear it rather than embrace it. This fear reached its peak in the later years of Númenor, where even the long life given to the Númenóreans was not enough, and wise men did all they could to try to escape death altogether. In the end, this desperation led to Númenor's destruction when Ar-Pharazôn led a battle fleet to the Undying Lands, falsely believing that they held the secret of everlasting life.