Iyo Shinno no Hen (伊予親王の変)
Imperial Prince Iyo, prince of Emperor Kanmu, was in his father Kanmu's favor while Kanmu was in living and his mother Yoshiko's elder brother FUJIWARA no Otomo was, being as Dainagon (a chief councilor of state), the second-ranking official of Daijokan (Grand Council of State) next to udaijin (minister of the right) FUJIWARA no Uchimaro and boasted of strong political power. During the era of the Emperor Heijo, he served as Nakatsukasa-kyo (Minister of the Ministry of Central Affairs) cum Dazai no sochi (Governor-General of the Dazai-fu offices) from 806 and was actually the leader of the Imperial Family. As he maintained a good relationship with his elder brother Emperor Heijo, he followed his elder brother on the occasion of an Imperial visit to Shinsen-en Temple in June 807, made an offering and participated in a party throughout the day.
In November of the same year, however, FUJIWARA no Otomo scented the information that FUJIWARA no Munenari was instigating Imperial Prince Iyo to rebel and reported to that effect to udaijin FUJIWARA no Uchimaro. At the same time, Imperial Prince Iyo also reported the details of Munenari's instigation to Emperor Heijo. Then, the Imperial court inquired of Munenari and Munenari confessed that Imperial Prince Iyo himself was a ringleader. Upon hearing this confession, Emperor Heijo ordered ABE no Anio and KOSE no Notari to arrest Fujiwara no Yoshiko and Imperial Prince Iyo and imprisoned them at Kawahara-dera Temple. Though two persons insisted on their innocence, it was not accepted and they poisoned themselves on December 18. They were found not guilty later on and their graves were designated as Imperial tombs.
Munenari was banished due to this incident and Fujiwara no Otomo, an uncle of Imperial Prince Iyo, was also banished to Iyo Province as an accomplice. Further, Chunagon (middle counselor) FUJIWARA no Takatoshi suffered the ripple effect of this incident and was removed from his position. As two high officials of the clan were punished due to this incident, the power of the Southern House of the Fujiwara clan declined substantially.
Although it is also said that Munenari was instigated by the siblings of Fujiwara no Nakanari and Kusuko, the details are not clear. However, it seems that the relationship between Emperor Heijo and Nakanari/Kusuko became stronger and the court rank of Naishi no Kami (Principal Handmaid) was upgraded from Jugoi (Junior Fifth Grade) to Jusanmi (Junior Third Grade) soon after this incident taking into account the promotion of Kusuko, who was then Naishi no Kami.