set on/upon someone
phrasal verb with set verb uk/set/ us/set/ setting | set
[ often passive ]攻击（某人）
to attack someone
He was set upon by a vicious dog. 他遭到了一只恶狗的攻击。
围困（某人）;堵截（某人）
to surround or catch someone and prevent them from escaping
As he left the theatre, the singer was set upon by fans desperate for autographs. 那名歌手在离开剧院时被疯狂的歌迷围住索要签名。