In the universe of demon slayer there is two sides. The demon slayers who’s job is to kill the demons and the demons who kill and eat humans to survive and grow more powerful. Demons can be killed by their head being cut off by a Nichirin blade or being exposed to the sun. Demons only come out at night due to their weakness of the sun which is when the skaters hunt and kill them. The skaters have a ranking system and the most powerful of them all are the Hashira who have killed the most demons