Demons in the Demon Slayer series have roamed the earth ever since the Heian Era, this was the era that Muzan turned into a demon because of an experiment gone wrong when he was trying to cure himself of a terminal illness.
Demons in the Demon Slayer series have roamed the earth ever since the Heian Era, this was the era that Muzan turned into a demon because of an experiment gone wrong when he was trying to cure himself of a terminal illness.