The was a mythological event in Japanese prehistory, related in sources such as the Kojiki and the Nihon Shoki. It relates the story of how the rulership of Japan passed from the earthly kami (kunitsukami) to the kami of Heaven (amatsukami) and their eventual descendants, the Imperial House of Japan.
Background
The Kojiki and the Nihon Shoki both relate that the Japanese archipelago were created by the primordial couple Izanagi and Izanami, who also brought forth many gods into existence, three of which – Amaterasu, Tsukuyomi and Susanoo – were appointed to govern the sky (Takamagahara, the 'Plain of High Heaven'), the night, and the seas, respectively.
Susanoo, expelled by Izanagi either because he refused to perform his allotted task of ruling the sea (Kojiki) or his impetuous nature (Nihon Shoki), went to Takamagahara to see his sister. Suspected of insurrection, Susanoo protested his innocence, at which the two gods underwent a trial by pledge, giving birth to five male kami (Amaterasu's sons) and three female kami (Susanoo's daughters) when each chewed and spat out an object carried by the other (Amaterasu Susanoo's ten-span sword, Susanoo Amaterasu's magatama beads).
Declaring himself winner of the trial, Susanoo then began to wreak havoc upon Takagamahara, causing Amaterasu to hide herself in the Ama-no-Iwato, plunging heaven and earth into darkness. Though Amaterasu was eventually persuaded to come out of the cave, Susanoo was banished a second time as punishment for his misdeeds. He then came down to Ashihara-no-Nakatsukuni (the 'Central Land of Reed Plains', i.e. the earthly land of Japan), to the land of Izumo, where he slew the eight-headed serpent Yamata-no-Orochi and married Kushinada-hime. At length, Susanoo went to the underworld (Ne-no-kuni) to become its ruler.
A son (Nihon Shoki) or descendant (Kojiki) of Susanoo, Ōnamuji, married the goddess Yagami-hime of Inaba Province, earning the jealousy of his eighty brothers, who were seeking for her hand in marriage. Seeking refuge in Ne-no-kuni after his brothers had made attempts on his life, Ōnamuji met Susanoo's daughter Suseri-bime, with whom he immediately fell in love with. Upon learning of their affair, Susanoo imposes four trials on Ōnamuji, each of which he overcame with Suseri-bime's help. Taking his new wife Suseri-bime, as well as Susanoo's sword, koto, and bow and arrow back with him, Ōnamuji – now called Ōkuninushi (大国主 'Master of the Great Land') – defeats his wicked brothers, thereby becoming the lord of Ashihara-no-Nakatsukuni.
Upon subduing his brothers, Ōkuninushi takes a third wife, Nunakawa-hime of Koshi, causing his second (now chief) wife Suseri-bime to become jealous. Ōkuninushi then begins the task of creating the land (kuni-zukuri) started by Izanagi and Izanami, being helped in his task by a dwarf named Sukunahikona, a son of the primordial deity Takamimusubi (Nihon Shoki) or Kamimusubi (Kojiki). Together they made the lands habitable and invented means of dispelling various diseases and calamities such as medicine and magic.
Plot
Ame-no-oshihomimi
In time, the amatsukami of Takagamahara, headed by Amaterasu or/and Takamimusubi, decided that Ashihara-no-Nakatsukuni, considered to be overpopulated by unruly and evil kami, must be turned over to them to be pacified. Amaterasu decreed that Ame-no-oshihomimi (天忍穂耳命), one of the five sons born to Amaterasu when Susanoo chewed her magatama beads, shall take possession of the earth and ordered him to go down to it. Ame-no-oshihomimi, inspecting the land below from the bridge connecting heaven and earth, deemed it to be too tumultuous and refused to go any further, instead going back to report what he saw.
Ame-no-hohi
The heavenly gods then decided to send another of Amaterasu's sons, Ame-no-hohi (天菩比神, 天穂日命), the most heroic among the gods, down to Ashihara-no-Nakatsukuni. Ame-no-hohi, however, began to curry favor with Ōkuninushi and did not send back any report for three years.
The Nihon Shoki adds that Ame-no-hohi's son, Ōsobi-no-mikuma-no-ushi (大背飯三熊之大人) was sent afterwards, but like his father, he did not report back to Takamagahara.