Disclaimer: I do not own Amaterasu or any other Okami characters, they are property of Clover. I also do not own Naruto, or any of THOSE characters either, they belong to Kishimoto.
Prologue
To defeat the evil consuming the Celestial Plains, the Sun goddess Amaterasu no Omikami used all of her available power. Once the evil was banished from the Plains, Amaterasu was once again returned to her dormant state encased inside the stone replica of Shiranui. Waka took the statue back to it's place in Kamiki village, where the village leader promised that he and his descendents would watch over it until the goddess was needed once again.
The Brush Gods, scattered themselves across the country once again, taking Amaterasu's weapons with them. Divine Retribution, Amaterasu's first mirror was kept by the village leader for a while, but almost 2 decades later it was given to a nearby ninja village for safe-keeping.
Yami, and his demonic followers slowly crept back into and spread across the Five Countries. The evil creatures wallowed in the evil and bloodshed created by the humans, and Yami stayed hidden, slowly growing more and more powerful.
Almost 500 years later, Amaterasu would be needed again. Her release would be attributed to the burning love in a child's heart when he came to her resting place, half-dead, to escape the bandits that were pillaging his village. He would beg for Amaterasu to come protect his village and his prayers would be powerful enough to awaken the sleeping goddess so she could once again defeat Yami and the evil power that he spread.