With Emperor Shoutoku's death, the Tenmu emperor's family who had been in power since Jinshin no ran (revolution) could not hold onto power.
Emperor Kounin, who was a grandson of Emperor Tenchi, became the new emperor in 770. He invited Igami Naishinou and Osabe shinou mother and child as Empress and Prince.
Under the new emperor everything went well but it is said that there was a 'curse' such that the Prince would soon become emperor.
Before long the Emperor and Empress were deposed and imprisoned.
Yamabe shinou became the Prince.
Yamabe shinou's father was Emperor Kounin and his mother was Takano Niigasa. Their family originated outside Japan and for that reason, at first, there were many objection to him become emperor.
However he was strongly supported by Momokawa Fujiwara who held considerable power at that time.
Finally in 781 when he was 45 years old, Yamabe shinou became emperor.