Legion means a large group or in another parlance it may mean "many". In the Christian Bible, it was used to refer to the group of demons, particularly those in two of three versions of the exorcism of the Gerasene demoniac, an account in the New Testament of an incident in which Jesus performs an exorcism.
Development of the story
The earliest version of this story exists in the Gospel of Mark, described as taking place in "the country of the Gerasenes". Jesus encounters a possessed man and calls on the demon to emerge, demanding to know its name – an important element of traditional exorcism practice. He finds the man is possessed by a multitude of demons who give the collective name of "Legion". Fearing that Jesus will drive them out of the world and into the abyss, they beg him instead to turn them into a herd of pigs, which he does. The pigs then rush into the sea and are drowned (Mark 5:1–5:13).
This story is also in the other two Synoptic Gospels. The Gospel of Luke shortens the story but retains most of the details...