“Those who had seen
What had happened
To the demoniac
And to the swine,
καὶ διηγήσαντο αὐτοῖς οἱ ἰδόντες πῶς ἐγένετο τῷ δαιμονιζομένῳ καὶ περὶ τῶν χοίρων.
There is something similar in Luke, chapter 8:36, while there is nothing like this in Matthew. Mark said that those who had seen what had happened (οἱ ἰδόντες πῶς ἐγένετο) to this demoniac, the one possessed by the devils or evil spirits (ῷ δαιμονιζομένῳ) told everyone. They reported, described, or related it to other people (καὶ διηγήσαντο αὐτοῖς) how the swine or the pigs (καὶ περὶ τῶν χοίρων) ran into the sea. There was nothing secret about this transfer of evil spirits from a human to a herd of pigs.