"Legion" are a group of demons referred to in the Christian Bible. The spirit who said "For we are many," in ancient scriptures. The name comes from the Roman military term for an army of 3,000 to 6,000 men."
While the types of demons that make up Legion is unknown, they are purportedly said to be the same kind of demon that would inhabit a host and would have a single hive mind. Speaking of which, the demons of Legion do act like a hive-mind, always speaking in unison, and thus always knowing what the other demon is thinking and preparing to do.
When gathered together, Legion can also expand its power far beyond the norm of the demons that make it up as a single unit. Legion can even take several shapes and forms that it is limited only to itself and to what it can reach. To the point where it can even take the form of an earthly animal, although the animal itself becomes severely grotesque having numerous hands, eyes, mouths, and arms sprouting across its body.
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.