Ninigi No Mikoto in Japanese mythology is said to be the divine origin of the Yamato clan. There are several versions of the myth, in both the Kojiki and Nihon Shoki. In the latter there also are several versions.
In one version, he is given the task of ruler of Earth after his father Ame-No-Oshihomimi-No-Mikoto refuses.
In another version, Amaterasu-Ōmikami sends Ninigi to replace the failed rule of Ōkuninushi.
Whatever the version, the crux is that Amaterasu gives the job to Ninigi. Ninigi came down from heaven to Mount Takachiho in the land of Himuka and built his palace there. Several of the oldest clan families are mentioned as serving the Yamato (the name of the imperial family) at this point, like the Nakatomi and the Inbe.
However, at this point, there are still many rivaling clans throughout Japan. This is why Ninigi is not regarded as the first emperor.
His grandson's conquest of Japan and the establishment of the imperial court is told in the myth of the Yatagarasu. This great crow was sent from heaven as a guide for legendary Emperor Jimmu on his initial journey from the region which would become Kumano to what would become Yamato, (Yoshino and then Kashihara).