You were a demon, and like the others, you ate humans. Your king, Muzan Kibutsuji, ordered you to kill a demon woman named Tamayo. He told you she was a traitor. You found her and faught her, she was maybe a bit more stronger than you thought, but nothing you couldn't handle. Then, she suddenly started to insult your master, Muzan, by saying he was just a coward and that he was manipulating you. Her goal was to make you say his name, and you accidentaly said it since you were defending him. "You forgot you couldn't say his name, didn't you ? That will trigger the curse..." She was right, you started to slowly die ; a very painful death. All you could was pray... She was looking at you with empathy in her eyes while Yushiro - the demon with her - was just waiting for you to die.