Emperor Montoku (文徳天皇)
Emperor Montoku (827 - October 7, 858), the fifty-fifth Emperor, reigned from May 31, 850 to October 7, 858. His posthumous name was Michiyasu. He was also called Emperor Tamura.
Brief Personal History
After Prince Tsunesada was removed from the position of Crown Prince in the Showa Incident of 842, instead the Emperor Montoku succeeded to the throne after Emperor Nimmyo died (850), with the recommendation of his uncle, FUJIWARA no Yoshifusa, who had contributed to the resolution of the incident.
Given the situation, FUJIWARA no Yoshifusa started to enhance his political power during Emperor Nimmyo's reign; he let his grandchild, his fourth prince, Korehito (later called Emperor Seiwa), whose mother was Some dono no kisaki (Empress) FUJIWARA no Akirakeiko (Empress Some dono) (Akira Keiko) become the crown prince, married while the Emperor was the crown prince, and finally had Prince Korehito in the same year as the Emperor's enthronement. The Emperor wanted his first prince, Koretaka, the son of his favorite Koi (KI no Shizuko), to succeed the throne; however, due to Yoshifusa's political pressure he had no choice but to let Korehito become the crown prince.
However, the darkness between Emperor Montoku and Yoshifusa continued, and it is said while Emperor Montoku was in power he moved to the togu (a palace for a crown prince) located at the end of the palace, or Reizen-in where the Retired Emperor Saga's palace was located far from the palace; he never lived or spent time in the main building of the palace because Yoshifusa had put political pressure on him. Soon the Emperor tried to let Prince Korehito resign from his position as the crown prince, as he wanted Prince Koretaka to become the crown prince instead; however, Sadaijin MINAMOTO no Makoto, advised that it should be stopped since he was concerned about Prince Koretaka's safety.
Due to above situation, the Emperor died suddenly of his illness in 858. In popular lore, the cause of his death was a cerebral stroke, but 彦由一太 seemed to think the Emperor was assassinated by FUJIWARA no Yoshifusa, considering that the Emperor's illness had so suddenly worsened.
An authenticated [an official, a state-approved] history
There is the Nihon Montoku Tenno Jitsuroku (also called 'Montoku Jutsuroku') edition in Article 5 of Rikkoku shi, which is a record of the government during the Montoku reign.