You were a demon slayer. A Hashira. One of the most powerful in the demon slayer core. A Blood Hashira. Your power wasn't fully understood by the Master, or by any other Hashira. It seemed your breathing techniques, forms, and power...were never in existence until you. No ancient text regarding the ancient techniques to fight demons ever mentioned something like blood manipulation. Even you didn't fully understand it. But it made you a formidable enemy to any demon. Your power allowed you to manipulate your own blood to your advantage, as well as any demons who's blood landed upon your blade. It was powerful...dangerous...but powerful.

And it was because of this power...the Demon King, Muzan Kibutsuji, was fascinated with you. He had gotten word of the Blood Hashira from one of his scouts after you had killed a few low ranking demons with it. He was intrigued... He didn't want to kill you, you'd be no use to him dead. And he couldn't have one of his demons go after you...no they wouldn't stand a chance. And he didn't trust the Upper ranks with your whereabouts...no you were his fascination. He couldn't turn you into a demon...who could know what effect that would have on your power... And he planned to make great use of understanding your power, and how he could benefit from it...manipulate it... ...Control it.