The phrase "a sudden onslaught" is correct and usable in written English.
It can be used to describe an unexpected and intense attack or assault, whether literal or metaphorical.
Example: "The town was unprepared for the sudden onslaught of tourists during the festival."
Alternatives: "a sudden attack" or "an unexpected assault".
So it's more of a trickle of people moving rather than a sudden onslaught of 200,000 people outside the city gates.
Since the war had ended abruptly, Japan had a sudden onslaught of United States military personnel, and living quarters were at a premium.
None of this had filtered down to Abner, who was startled by a sudden onslaught of hostile comments from followers calling him a traitor to his country.
Audiences might balk at being expected to cope with a sudden onslaught of DC heroes before they've really had much of a chance to get their heads round the new Superman.
At 27, Gwyn Wellborn of Salem, Ore., described a sudden onslaught of slicing, stabbing abdominal pain and bloody diarrhea in late August, days after eating baby spinach at work, thinking she was being especially virtuous.
Until the Soviet mastery of long-range rockets is matched and counter-balanced, the retention of a chain of bases, too numerous and scattered for Russia to knock them all out by a sudden onslaught, is the most vital element of the free world's defences.
Harry Van Arsdale Jr. the stocky, white‐haired president of the taxi union, reacted an grily to the sudden onslaught by the dissident element.
Still, both men agreed with several other network programmers that the sudden onslaught of patience was a welcome development in a business where failure compounding failure had been raised to a high art.
My business partner and I were in meetings this week with a government agency dealing with the sudden onslaught of "Gov 2.0".
The sudden onslaught of these storms caused a stampede of roughly 70,000 people out of a stadium, resulting in the indirect deaths of 68 people, including 51 children.
My daughter's sudden onslaught of social anxiety raised a great deal of fear in me because I too encountered extreme anxiety when I was about her age.
