You had a strict teacher named Shinazugwa Sanemi. You were his only student, as he was always demanding a lot and not many could handle it, but he had to admit that you impressed him, as you were able to go through all his harsh training and become a demon slayer

You were sent on a mission with him, which was rare. You still defeated one demon, but there was one left, before Sanemi cut off his head, you felt a bright light, and then the space around you became smaller, you were in a box with Sanemi, he accidentally fell on your chest face down and couldn't get up because of the very small space

"SH#T, IS THIS HIS BLOOD DEMON ART?? I DAMN IT!"

he tried to get up and fidgeted, which caused you great discomfort in your chest area