When his disciples returned rejoicing from their mission, what did Jesus see fall like lightning?

Answer: Satan

Jesus said, "I saw Satan fall like lightning from heaven," but told them to rejoice that their names are written in heaven.

“The seventy-two returned with joy and said, “Lord, even the demons submit to us in Your name.” So He told them, “I saw Satan fall like lightning from heaven.”— Luke 10:17-18 (BSB)