The onslaught of Mordor broke like a wave on the beleaguered hills, voices roaring like a tide amid the wreck and crash of arms.
In that dale behind the Morannon were the tunnels and deep armouries that the servants of Mordor had made for the defence of the Black Gate of their land; and there now their Lord was gathering in haste great forces to meet the onslaught of the Captains of the West.
There are no more uses of "onslaught" in the book.
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They withstood the onslaught and counterattacked.
Her campaign did not survive an onslaught of attack ads.