[Legolas]

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Eru granted immortality (or mortality). It was given or taken away at his sole discretion.

From Letter No. 131:

The making, and nature, of the Children of God, were the two chief secrets. All that the gods knew was that they would come, at appointed times. The Children of God are thus primevally related and akin, and primevally different. Since also they are something wholly 'other' to the gods, in the making of which the gods played no part, they are the object of the special desire and love of the gods. These are the First-born, the Elves; and the Followers Men. The doom of the Elves is to be immortal, to love the beauty of the world, to bring it to full flower with their gifts of delicacy and perfection, to last while it lasts, never leaving it even when 'slain', but returning - and yet, when the Followers come, to teach them, and make way for them, to 'fade' as the Followers grow and absorb the life from which both proceed. The Doom (or the Gift) of Men is mortality, freedom from the circles of the world. Since the point of view of the whole cycle is the Elvish, mortality is not explained mythically: it is a mystery of God of which no more is known than that 'what God has purposed for Men is hidden': a grief and an envy to the immortal Elves.

'gods' above refer to the Valar, 'God' refers to Eru

Later, in that same letter:

The doom or gift of God, of mortality, the gods of course cannot abrogate

Letter No. 153:

Immortality and Mortality being the special gifts of God to the Eruhini (in whose conception and creation the Valar had no part at all) it must be assumed that no alteration of their fundamental kind could be effected by the Valar even in one case: the cases of Lúthien (and Túor) and the position of their descendants was a direct act of God. The entering into Men of the Elven-strain is indeed represented as part of a Divine Plan for the ennoblement of the Human Race, from the beginning destined to replace the Elves.

Letter No. 156:

...for the point of view of this mythology is that 'mortality' or a short span, and 'immortality' or an indefinite span was part of what we might call the biological and spiritual nature of the Children of God, Men and Elves (the firstborn) respectively, and could not be altered by anyone (even a Power or god), and would not be altered by the One, except perhaps by one of those strange exceptions to all rules and ordinances which seem to crop up in the history of the Universe, and show the Finger of God, as the one wholly free Will and Agent.

[ October 31, 2002: Message edited by: Legalos ]