于是乎 is a literary or formal connective meaning 'and then', 'thereupon', or 'thus'. It is a strengthened form of 于是 (yú shì) with the classical particle 乎 adding a slightly emphatic or rhetorical tone. You mainly encounter it in written Chinese, classical-style narratives, or speeches — not in everyday casual conversation. It signals that what follows is a natural consequence or the next step of a story, much like 'hence' or 'so then' in English.