18 December 876 : In the 18th year of Emperor Seiwa's reign, he abdicated. Prince Sadaakira received the succession (senso).[6]
877 (Gangyō 1): The beginning of a new nengō was proclaimed the beginning of Yōzei's reign.[6]
20 January 877 (Gangyō 1, 3rd day of the 1st month): Yōzei was formally established as emperor (sokui).[6] This was confirmed in ceremonies.[7]
883 (Gangyō 7): Yōzei showed signs of mental illness and violent incidents which could not be ignored. For example, he killed people randomly in the court.[8]
4 March 884 (Gangyō 8, 4th day of the 2nd month): Fujiwara no Mototsune removed Yōzei from the palace; and Yōzei was deposed as emperor.[9]
Yōzei was succeeded by his father's uncle, who became known as Emperor Koko. During the reign of Kōkō's son, Emperor Uda, the former emperor became dangerous again.
889 (Kanpyō 1, 10th month): Former- Emperor Yōzei again began killing people randomly. Sometimes he disappeared into the mountains where he chased wild boars and Sika Deer,[10]
Yōzei lived in retirement until the age of 81.[11]
After his death
The actual site of Yōzei's grave is known.[1] This emperor is traditionally venerated at a memorialShintoshrine (misasagi) at Kyoto.
In ancient Japan, there were four noble clans, the Gempeitōkitsu (源平藤橘). One of these clans, the Minamoto clan (源氏) are also known as Genji, and of these, the Yōzei Genji (陽成源氏) are descended from the 57th emperor Yōzei.
Yōzei fathered nine sons who were born after his abdication.[13]