The trope of a person being possessed by multiple demons who speak with one voice is from the New Testament of the Christian Bible. Matthew 9: 28 - 32, and Mark 5: 1 - 13. These passages appear to describe the same incident although in Matthew there are two possessed men and in Mark only one man is mentioned.

"Then he asked him "What is your name?" and he answered "My name is Legion, for we are many" and the unclean spirits were driven out of the man and allowed to enter a large herd of pigs which rushed down a steep place into the sea and drowned.