In Takamagahara, the realm of the Gods, there was the Goddess of the Sun and sole Queen of the gods, Amateratsu and her husband, the God of the moon, Tsukiyomi , they soon fell in and out of love but we're still married when they gave birth to a boy and named him Jimmu who would later become the first emperor of Japan. 

During the period of his eternal youth ,although Jimmu had grown to be kind and loving, he also grew to be an egostical and arrogant god who had the power to destroy dimensions and due to this he defied his father and destroyed earth's second moon leaving only one and soon after his father, Tsukiyomi stripped him of the powers of the moon and then cursed him to be human , therefore he was cast on to the earth because humans can't live among Gods. 

Amateratsu though disappointed in her so loved him dearly and let him keep the powers of the sun and renewed his immortality but still keeping him human.

Jimmu crossed the Ame-no-ukihashi bridge to earth and ended up in Yamato, Japan with no return to the realm of the Gods , as soon as he landed , the people of Japan recognized and hailed him as their leader.

After Jimmu was recognised as a blessing the Gods cast upon on the earth to save it , they held a coronation ceremony where they had drinks and food that could feed the earth itself, music from the best musicians, murals of Jimmu by the best artists, the brightest firework sent up for many days on end and people danced and sang for many days on end to celebrate the inauguration of their first ruling emperor.

Thanks to their show of respect to the Gods , Tsukiyomi and Amateratsu blessed Japan because of their decision to make Jimmu emperor.

But Tsukiyomi cursed the royal family, that when an heir comes of age and their powers have fully manifested, the sitting emperor and empress shall die because he didn't want them to be able to rule forever, especially Jimmu.

Japan became so powerful that one of Jimmu's further down descendants, Emperor Keiko increased Japan's territory by taking over the Australias and the islands surrounding Japan peacefully. They were given to him as a gift from Amataretsu.

The immortality that Jimmu was blessed with made his will to be passed down from generation to generation , therefore every emperor after Jimmu who was rightfully put on the throne carried out his will.

This is the story of how that will died.