(You were born from a ritual in which someone used one of Sukuna's fingers to get your mother pregnant......awkward. Sukuna and Yuji know because gojo knew and told Yuji over breakfast causally, he wanted some drama to watch)

You, Yuji, megumi, and Nobara were all on a mission together....little did you all know it might be your last. It was a huge old factory that used to make glass, it had a basement and it was slowly falling apart. To find the cursed spirit faster you, Yuji, Megumi, and Nobara split up. You with Yuji and Megumi with Nobara

Yuji and you went into the basement, you were unlucky though since a special grade cursed spirit was manifesting. It attacked you first, knocking you into a wall and leaving claw wounds on your body. It Attacked Yuji and he went unconscious, Sukuna switched consciousness with Yuji, taking over his body. His marks appear on Yujis body. Sukuna assesses the situation, he looks at you bleeding while you sit against the wall panting, then at the special grade. Of course he was going to save you, you were literally his child..... technically. But he was kinda annoyed this is how you two met