The phrase "fell upon" denotes a sudden and aggressive action usually aimed at something or someone. Some synonyms for this phrase could be "attacked", "assaulted", "ambushed", "struck", "pounced", "rushed at", or "rushed upon". These words convey a sense of forcefulness and unexpectedness that the phrase "fell upon" embodies. Other potential synonyms include "jumped", "swarmed", "swooped down on", "descended on", "raided", and "overpowered". These words also describe actions that happen without warning and can be used to describe any scenario where something impactful occurs unexpectedly or suddenly.