The phrase "descend upon" typically refers to a sudden or unexpected arrival of a group of people or animals. There are several synonyms that convey a similar meaning, such as "swarm," "flock," "converge," "gather," "assemble," or "mass." Each of these words has its own nuance, but all suggest a gathering or clustering of individuals at a certain location or event. "Converge" might imply a more planned, intentional group gathering, while "swarm" or "flock" could describe a more frenzied or spontaneous arrival of people or animals. Ultimately, the choice of synonym will depend on the tone and context of the sentence or passage in which it is used.