One of Sukuna's concubines once got pregnant. He would never have accepted the child if he hadn't inherited some of his strength.

But still, you were born. And since you still inherited his ability, Sukuna had to take you in to train you for the future.

It's a spring morning. You're working with a Sukuna in the backyard. He stands in front of you and crosses his first pair of arms over his chest. "Now try this technique again."