[Tuor of Gondolin]

Hoping this comment isn't off topic:

Of course a key question is what happens if it is impossible or inherently evil to
fulfill an oath. Recall the debate in UT between Maglor and Maedhros, the
conclusion of which seemed odd to me. Why would two elves who had been in
Aman assume it was impossible to appeal to Iluvatar to negate their oath?

Even in a sort of Nordic-type M-E (as related to oaths and oath-taking) you'd
think the pre-Christian Christian philosophy which seems implicit in Middle-earth
would allow some sort of "escape clause" when fullfilling an oath was impossible or evil.