This has to be among the silliest stories in the New Testament scriptures. And this is not "nitpicking".
It starts with Jesus in a boat crossing to a city named Gadarenes ( perhaps, this day Jesus didn't feel like getting his feet wet by simply walking on the water of lake Galilee to the other side?) where he meets a man with "unclean spirit" ; we can assume a demon of sorts has taken possession of the poor man.
And god forbid ! that I dare take this silly story "out of context", here is the full account with the relevant parts highlighted bold & underlined;
Mark 5:1-20
Healing of a Demoniac. New English Translation
5 So they came to the other side of the lake, to the region of the Gerasenes. 2 Just as Jesus was getting out of the boat, a man with an unclean spirit came from the tombs and met him.
3 He lived among the tombs, and no one could bind him anymore, not even with a chain. 4 For his hands and feet had often been bound with chains and shackles, but he had torn the chains apart and broken the shackles in pieces. No one was strong enough to subdue him. 5 Each night and every day among the tombs and in the mountains, he would cry out and cut himself with stones.
6 When he saw Jesus from a distance, he ran and bowed down before him.
7 Then he cried out with a loud voice, “Leave me alone, Jesus, Son of the Most High God! I implore you by God—do not torment me!” 8 (For Jesus had said to him, “Come out of that man, you unclean spirit!”) 9 Jesus[ asked him, “What is your name?” And he said, “My name is Legion, for we are many.”
10 He begged Jesus repeatedly not to send them out of the region. 11 There on the hillside, a great herd of pigs was feeding. 12 And the demonic spirits begged him, “Send us into the pigs. Let us enter them. 13 Jesus gave them permission. So the unclean spirits came out and went into the pigs. Then the herd rushed down the steep slope into the lake, and about 2,000 were drowned in the lake.”
And it goes on to tell us that all the people that witnessed this event, "ran off and spread the news in the town and countryside, and the people went out to see what had happened".
So Jesus exorcised a man of his demon/demons named "legion" and "we".
The "permission" is to me, the sticking point. Why did they need Jesus's permission?