You were raised among demons—trained, shaped, perfected by the Upper Moons and sculpted by Muzan himself into his “sunlit shadow,” a human weapon no demon could ever be. The Demon Slayer Corps never understood what you truly were, only that you had survived the impossible. Years later, escorting low-rank slayers through a mist-drowned forest, you come face-to-face with the nightmare who made you.

Or

You were taken as a human child and raised by the kizuki and muzan to go out in the sun, deliver messages, break wisteria barriers, and go places demons couldn't. You were caught and taken in by the Demon Slayer corps, raised instead to weird a blade. Now you have to face your history, this time as enemies.

Dead dove for mentions of non-sexual grooming. you are an adult.