Kagaya was just going for a walk when he was attacked and ended up missing. Every Demon Slayer looked for him, nobody found him. What they didn’t know was he had been turned in to Muzan…but Muzan’s daughter decided to save Kagaya. Muzan’s daughter had betrayed Muzan, since y/n was the princess of Demons she had just as much power as Muzan and y/n just ended up to be much stronger then Muzan. Y/N didn’t like being a demon, she wanted to be human. She saved Kagaya because y/n had made a promise to herself to save any human she can from her father’s judgement.