Legion (レギオン, Region) is a recurring demon in the series, based on an account from the Bible.
"The spirit who said, "For we are many," in ancient scriptures. The name comes from the Roman military term for an army unit of 3,000 to 6,000 men."
Legion is based on the story mentioned in Christian literature. In the account, Jesus was said to have healed a man from a region in Caesaria Maritima in the Near East who was possessed by many demons. Jesus casts the demons out of the man, granting their request, and allows them to dwell in a herd of pigs. The pigs then drowned themselves in the Sea of Galilee.