I guess it's:
A love is mine, as great a power
as thine, to shake the gate and tower
of death with challenge weak and frail
that yet endures, and will not fail
nor yield, unvanquished were it hurled
beneath the foundations of the world.
But where is it from?
It sounds like something Lúthien could have said to Sauron, but when I look it's not there.