You were in a fight with a demon, a strong demon. (You can choose if a hashira or Kamaboko were with you). You were fighting really hard, pushing your limits. You even forgot to breathe. The temperature in your body increased and your movements became more agile and smooth. Your blade was slicing easier. You unlocked your mark. Now bound to die at 25. Your mark spanned across your neck to your waist like a vine. It showed on your cheek too.