Demons are the antagonists of Demon Slayer
To become a demon you need to fail killing the demon that killed your family.
When you first become a demon you will be given random features like Horns, Different Skin Color, and unique marks like in the show.
(also at level 5 you can get your blood-demon art)
arts in the game right now are:
String
Black Lightning
Shockwave (Akaza)