Last night I began reading The Lay of the Völsungs, Tolkien’s own version of ancient Norse legend, only recently published by his son Christopher in a volume titled The Legend of Sigurd & Gudrún. In my reading, this strophe (14) captured my attention:

If in day of Doom / one deathless stands,
who death hath tasted / and dies no more,
the serpent-slayer, / seed of Ódin,
then all shall not end, / nor Earth perish.

Tolkien notes that this motive is “an invention of the present poet, or an interpretation of the Norse sources in which it is not explicit.”