Mark, the writer, wants his readers to know that Jesus has power over satan and his demons. This is made clear by the response of the demon when he saw Jesus in Mark 5:7. Demons quake at God. Other verses will be used below to show that Jesus possessed power over satan and his demons.
Mark tells a story of a demon-possessed man. The man was possessed by more than one demon, but only one demon spoke who is the authoritative figure in the story. The demon tells Jesus in 5:9 that his name is Legion; for we are many. This term suggests that the man was possessed by numerous demons. In 5:7, the demon was speaking using the voice of the possessed man. He said to Jesus,” What business do we have with each other…?” Similar expressions are found in the O.T. where they mean, “Mind your own business!” The demon sensed he was to be punished and used the strongest basis for an oath he knew, “I implore You by God, do not torment me!” The demons were fearful of being sent into eternal judgment. Jesus consented to the demon’s request, and the pigs carried the demons into the sea. After Jesus healed the demonic man, witnesses were frightened by Jesus’ power.
The meaning of this verse is that Jesus is the Son of God who possesses power over satan and his demons. Within the biblical narrative, the reader sees how much power Jesus has by the great number of swine that are destroyed. Jesus’ exorcism results in awe and astonishment.