The expression "eight undocumented eras" ("kesshi hachidai" in Japanese) refers to the eight emperors from Emperor Suizei (second emperor) to Emperor Kaika (ninth emperor) whose pedigree records (called "Teiki" in Japanese) exist but whose achievements are not recorded in "Kojiki" and "Nihon Shoki" and the period represented by these emperors. It is widely accepted among scholars in the post-war period that these emperors did not in fact exist but are imaginary characters created at later times.
The emperors of Kesshi-Hachidai
Emperor Suizei - Kamununakawamimi no Sumeramikoto
Emperor Annei - Shikitsuhikotamatemi no Sumeramikoto
Emperor Itoku - Oyamatohikosukitomo no Sumeramikoto
Emperor Kosho - Mimatsuhikokaeshine no Sumeramikoto
Emperor Koan - Yamatotarashihikokunioshihito no Sumeramikoto
Emperor Korei - Oyamatonekohikofutoni no Sumeramikoto
Emperor Kogen - Oyamatonekohikokunikuru no Sumeramikoto
Emperor Kaika - Wakayamatonekohikoobibi no Sumeramikoto
It is generally believed that these eight generations of emperors are fictive characters created for the pedigree record based on the Chinese Shinyu (year of the Metal-Rooster ("Kanototori" in Japanese) in the Chinese sixty-year calendar cycle) revolution philosophy in order to make the origin of the Imperial Family look more ancient than it actually was.
Kanototori is one of the zodiac signs in Chinese astrology and is said to be the year of revolution in China. Ancient Chinese legend has it that a great revolution occurs in the twenty-first Kanototori year in the astrological calendar. According to this legend, 601 A.D., when Prince Shotoku took office (although he is believed to have taken office at the end of the 5th century, there is no record of his activities before 601), was the year of the great Shinyu revolution. Therefore, by subtracting 1260 (21 times 60) from 601, one is led to conclude that 660 B.C. (note that there is no such year as 0 A.D.) is the year when Jinmu was entrhroned as the first emperor.
Emperor Sujin (10th emperor) is also known as Hatsukunishirasu Sumeramikoto and this name (literally meaning the "first emperor who ruled the country") implies that he was the first emperor to govern the country. This leads us to believe that Emperor Suijin was the first empeor in the original pedigree and that the preceding generations of emperors were added to the pedigree at some later point in time.
The pedigree of the eight generations of emperors starting from Obiko no Mikoto is inscribed on the blade of a gold-inlaid sword excavated from the Inariyama-kofun tumulus in Saitama Prefecture in 1978. Ohiko no Mikoto that appear in this pedigree is estimated to be the general (Ohiko) mentioned in "Kojiki" and "Nihon Shoki" who conquered four countries - i.e. the Imperial Prince of Emperor Kogen (eighth emperor), but there is nothing inscribed on the sword blade that shows his relationship to the emperor. It implied that the existence of such emperors (kings) wasn't established in the genealogy when the iron sword was made.
The names of the fourth and sixth to ninth generations of emperors are obviously Japanese-style posthumous names, but according to reliable descriptions in "Kojiki" and "Nihon Shoki," Japanese-style posthumous names came to be used around the mid-sixth century; and although names like Jinmu and Suizei may sound like names of emperors used when they were alive, names that contain characters representing god (as in Jinmu) is unlikely to have been used as names of living persons, which leads us to believe that these were names for legendary characters or something similar to posthumous names and that, therefore, these emperors were added to the imperial lineage later in history.
There are also inconsistencies about the mausoleums of these eight generations of emperors. There are huge burial mounds consistent in appearance with the archaeological chronology at the locations of mausoleums built for Sujin, the tenth emperor, and later. However, burial mounds for Kaika, the ninth emperor, and earlier are either burial mounds that are likely to have been built in later periods from an archaeological viewpoint or natural hills. There were no tumuluses that could possibly have been built around that time (early to mid-Kofun period (tumulus period)), nor were there any grave mounds from the Yayoi period.
The only records available about these emperors are descriptions about their pedigrees and there are no descriptions of their actual achievements, except for a story about how Emperor Suizei ascended to the throne by crushing the rebellion of Tagishimimi. This fact strongly suggests that their pedigrees were created out of materials unrelated to historical facts.
All cases of succession observed among these emperors are from father to son and no case of succession between brothers is found. These cases run counter to the actual historical development, since succession between brothers was replaced by father-to-son succession in a much later period.