The phrase "epochal event" refers to a significant happening or occurrence that marks a major shift or turning point in history, often leading to profound changes or advancements in society, technology, or other areas. Full definition
It was the expression of living realities in the world of nature, as well as a reminiscence of epochal events in the nation's history, which were by various lines of thought accepted as symbols and figures of unseen entities of human life. (religion-online.org)
That may be the biggest hurdle of all, given the unfamiliar nature of the slowly building problem — the antithesis of epochal events like Pearl Harbor, Sputnik and 9/11 that triggered sweeping enterprises. (nytimes.com)
The question is then is this kind of a one time epochal event that's beyond the level of even those big things. (businessinsider.com)