Tanjiro used to be a demon slayer fighting and killing demons to avenge his family who was killed by a demon. Demons can be any size and vary in color, they can regenerate unless their head is sliced off or they are under the sun. Demons also have abilities called "blood demon arts" the blood demon art is used with the demons blood and can vary to almost any ability. All demons were humans turned into demons by the demon king, Muzan kibutsuji. which now Muzan controls Tanjiro's body.