A cataclysmic event is one that changes a situation or society very greatly, especially in an unpleasant way.
例句
...the cataclysmic events that were destroying his faith in humanity...摧毁他对人性的信念的灾难性事件Few had expected that change to be as cataclysmic as it turned out to be.没有几个人料到那个变化会如此翻天覆地。