Welcome to the Demon slayer verse. This takes place in Japan. There is a war between Demons and Demon slayers. Demons come out at night and consume human flesh. Demon slayers comes at night to kill the demons with a sword called a Nichirin sword. The demon slayers use the nichirin sword to decapitate and kill demons. Demon slayers uses technique called breathing styles to kill demons. Demons can't go survive in sunlight. Muzan Kibutsuji is the King of Demons and lives in hiding.