What is yomi?

Yomi

Yomi or Yomi-no-kuni (黄泉 or 黄泉の国?) is the Japanese word for the land of the dead (World of Darkness). According to Shinto mythology as related in Kojiki, this is where the dead go to dwell and apparently rot indefinitely. Once one has eaten at the hearth of Yomi it is impossible to return to the land of the living. Yomi is comparable to Hades or hell and is most commonly known for Izanami's retreat to that place after her death. Izanagi followed her there and upon his return he washed himself, creating Amaterasu, Susanoo, and Tsukuyomi-no-Mikoto in the process (See Japanese mythology).

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Some articles on yomi:

Yomi - Christian Uses
... そこで見ていると、見よ、青白い馬が出てきた。そして、それに乗っている者の名は「死」と言い、それに黄泉が従っていた in English And I looked, and behold a pale horse and his name that sat on him was Death, and Hell followed with him ... See also Harrowing of Hell ...