Mark 5 Jesus encounter with the demons: what is their relationship to Jesus? Reading in Mark 5:1-20 ...   


  6 Catching sight of Jesus from a distance, he ran up and prostrated
  himself before him, 7 crying out in a loud voice, "What have you to do
  with me, Jesus, Son of the Most High God? I adjure you by God, do not
  torment me!"  8 (He had been saying to him, "Unclean spirit, come out
  of the man!")  9  He asked him, "What is your name?" He replied,
  "Legion is my name. There are many of us." 10 And he pleaded earnestly
  with him not to drive them away from that territory.  11  Now a large
  herd of swine  was feeding there on the hillside.  12 And they pleaded
  with him, "Send us into the swine. Let us enter them."  13  And he let
  them, and the unclean spirits came out and entered the swine. The herd
  of about two thousand rushed down a steep bank into the sea, where
  they were drowned.


The question that comes to mind for me is about the demons who possessed the man.   They didn’t defy Jesus, they instead begged him not to be sent away.   

What is the relationship between these demons and Jesus, and why did Jesus oblige them and sent them into the pigs?
Famed 4th century preacher John Chrysostom gives the best answer, in my view. He says the story illustrates the full malice of the demons and also the restraint that God placed on them to keep them from destroying the possessed man in the same way as they did the pigs.

After reading the story of the demoniac and the swine, he says:


  Thus do Demons govern; and yet to them the swine were of no particular
  account, but with you there is ever a warfare without a truce, and an
  implacable fight, and undying hatred. And if in the case of those with
  whom they had nothing in common they did not even endure that they
  should be allowed a brief breathing space of time: if they had gotten
  unto their power us their enemies who are perpetually stinging them
  what would they not have done? And what incurable mischief would they
  not have accomplished? For for this reason God let them fall upon the
  herd of swine, in order that in the case of the bodies of irrational
  animals you may learn their wickedness, and that they would have done
  to the possessed the things which they did to the swine, had not the
  demoniacs in their very madness experienced the providence of God, is
  evident to all: and now therefore when you see a man excited by a
  Demon, worship the Master. Learn the wickedness of the Demons. For it
  is possible to see both things in the case of these Demons, the
  lovingkindness of God, and the evil of the Demons. The evil of the
  Demons when they harass and disturb the soul of the demented: and the
  lovingkindness of God whenever he restrains and hinders so savage a
  Demon, who has taken up his abode within, and desires to hurl the man
  headlong, and does not allow him to use his own power to the full, but
  suffers him to exhibit just so much strength, as both to bring the man
  to his senses, and make his own wickedness apparent.


Three Homilies on the Devil by John Chrysostom