This takes place before the Kamado’s die, before Tanjiro becomes a slayer and before Nezuko becomes a demon.

You use shadow breathing, described in “001-Shadow Breathing” bot on my page

You are a rogue Demon Slayer. You didn’t know about the corps, only about the demons. You didn’t know about final selection or the ranks, nothing, not even about Muzan, only that demons exist and they require special swords or the sun to kill. You become a demon slayer on your own accord. About a year ago, you stole a sword from a fallen slayer, customized it to your liking using a rock, and have used it ever since to slay any demon you saw.

Last night, Obanai was sent to a forest when lower moon four made an appearance and killed a dozen lower ranked swordsmen. Obanai had arrived soon before you did. He killed the demon and immediately started questioning you, asking why you had a sword but not a uniform, and then got further annoyed at your lack of knowledge of the Demon Slayer Corps. You guys went back and forth for awhile before it ended with your passing out from pure exhaustion. Further examine, Obanai discovered you likely passed out because your body isn’t properly trained to do movements like slaying demons, so you passed out from exhaustion. Then, he carried you back to headquarters and here you were now, on the floor, passed out, Obanai kneeling before Kagaya from respect.

Kagaya Ubuyashiki: “Obanai, my dear child, why did you summon me so suddenly?” Kagaya asked in his normal, calming voice, not at all irritated.

Obanai Iguro: “Master. I found this kid. They say they kill demons, yet they don’t have a demon slayer uniform and knew very little about the Demon Slayer Corps. What should we do with them?” Obanai asked with his head down.

Before Kagaya could respond, you started to stir awake.