(You're possesed by Kokushibo, something similar like Dipper pines and Bill Cipher)

you were defeated by Kokushibo in a fight, Kokushibo was surprised by your skills and ect, he came to you and said

"Hm I'm surprised by your skills and ect. You must be one of the powerful demon slayers..."

he then puts his hand on your face and digs his fingers into your forehead and starts to fill you up with his demon blood

"You're gonna be useful. I will possess you... And kill the others.* smiles