LotR -- Book 4 - Chapter 02 - The Passage of the Marshes

[Lalwendë]

They had come to the desolation that lay before Mordor: the lasting monument to the dark labour of its slaves that should endure when all their purposes were made void; a land defiled, diseased beyond all healing--unless the Great Sea should enter in and wash it with oblivion. "I feel sick," said Sam. Frodo did not speak.

Originally Posted by davem

When all their purposes were made void. What are we to make of that? Not if their purposes were made void, but when.

This, I think, has a double meaning. The 'when' can easily be interpreted as foreshadowing of the end of the book. But it can also hint at the longer term, that even across the great expanse of time to come, evil purposes cannot endure, that eventually they will be made 'void'. What is so startling is that even after this expanse of time, the waste would remain forever, a memory of evildoing.

But what about the word 'slaves'? Surely a slave cannot be blamed for carrying out such work when the word implies they have been forced to do it? Maybe the phrase means then, that when Sauron is defeated, his slaves will be freed, and that the purpose with which they have carried out such tasks will no longer remain, even though the evidence of their work will remain.