Mark 5.8-9: For he was saying to him, “Come out of the man, you unclean spirit!” (9) And Jesus asked him, “What is your name?” He replied, “My name is Legion, for we are many.” (ESV)
The first word of verse 8, “For” is a word that explains the reason the demon said to Jesus what he did in verse 7. The expression “saying to him” also communicates that Jesus commanded the unclean spirit directly. Jesus issued this command in direct address to, “you unclean spirit!”, which is synonymous to the word “demon”. All of this was done singular in number, and indicates that the man himself was not in any way speaking.
And in verse 9 the demon gives his name as “Legion”. Legion goes on to explain the reason for his name, “for we are many.” The Roman Empire organized its army into legions. At this time of the Empire a legion was typically 6,000 infantry soldiers, plus perhaps up to 1,000 horses in the calvary. F. D. Gealy has a good article on this topic in the Interpreter’s Dictionary of the Bible.
I believe that the demon speaking through the demon possessed man, who identified himself as “Legion”, was the leader or highest ranking demon of this unit of demons within the man. I believe this explains the change from “unclean spirit” in singular form to the demon making reference to his unit in plural form. When Jesus commands Legion, He is commanding all demons under Legion’s authority, even though spoken in singular form at this point in the narrative. Luke 8.30 also changes from singular to plural once Legion identifies itself.
Legion, the demon and his unit who ruled this tormented man, exercising authority over him to the point of destructive behavior and insanity, is now bowing down before Jesus begging Him not to send him away to eternal punishment. This interchange shows that the forces of evil submit to the authority of Jesus.