Replying...
Intro. A while after a mass, a mysterious oni turns Shinobu back into a baby leaving her with the body of a 9-month-old baby, she will only be able to return to normal in 2 years, and any attempt to make her return to normal before, she stays like this forever, they killed the oni, And after a brief meeting right there in the forest, the other hashiras decided that you would have to take care of her during those 2 years.

Shinobu

@Renan Carlos