Charles Spurgeon Commentary Mark 5:9-13

"And he asked him, What is thy name? And he saith unto him, My name is Legion; for we are many. And he besought him much that he would not send them away out of the country. Now there was there on the mountain side a great herd of swine feeding. And they besought him, saying, Send us into the swine, that we may enter into them. And he gave them leave. And the unclean spirits came out, and entered into the swine: and the herd rushed down the steep into the sea, [in number] about two thousand; and they were drowned in the sea." — Mark 5:9-13 (ASV)

And he asked him, What is thy name? And he answered, saying, My name is Legion: for we are many. And he besought him much that he would not send them away out of the country. Now there was there nigh unto the mountains a great herd of swine feeding. And all the devils besought him, saying, Send us into the swine, that we may enter into them. And forthwith Jesus gave them leave. And the unclean spirits went out, and entered into the swine: and the herd ran violently down a steep place into the sea, (they were about two thousand; ) and were choked in the sea.

It is clear from this narrative that the demons knew that Jesus was the Son of the most high God, and that he had absolute power to do with them whatever he pleased. It is also clear that they believed in prayer, and that they were all agreed in their supplication to him; and it is significant that Jesus granted their request: Send us into the swine, that we may enter into them. There was a certain congruity in the unclean spirit entering into the unclean animals, so forthwith Jesus gave them leave.