A four-character idiom meaning 'even the prosperous inevitably decline'; a Buddhist-origin phrase expressing the impermanence of worldly success (sic transit gloria mundi).
歴史を見れば、盛者必衰の例は数多く存在する。
Looking at history, there are many examples of the prosperous inevitably declining.
彼は一時的に大成功したが、やがて盛者必衰を思い知らされた。
He had temporary great success, but was eventually reminded that the prosperous inevitably decline.