The event to which I was referring does not occur during the War of the Ring.
I feel a bit bad since these are good answers, and I probably should have thought this through a bit more thoroughly. I believe if I change the wording slightly, the riddle is better:
Soon will come your end,
Although the future may change.
Choose not hopeful paths.
The second line should be interpreted as a "this will definitely happen, unless something specific happens" event, as opposed to general uncertainty. This excludes bringing the Ring to Mount Doom as it was theoretically possible to defeat Sauron without destroying the ring (e.g., allowing Aragorn to wield it), though no options were particularly tasteful. Also, it was theoretically possible (though highly, highly unlikely) for the armies of Men to defeat Sauron at the Black Gate.
Hopefully this is now sufficiently exclusive, and I apologise profusely for the inordinate number of false positives.