Hanuda Village, landlocked and surrounded on three sides by mountains. The resounding siren and earthquake at midnight cause the village to become trapped in the Other World. A land torn to pieces, warped, and enclosed by a sea dyed red... What are the red sea and the curse on the village? Its frightening rules are disclosed here.
The story of Siren starts more than 1300 years ago, with a female villager eating a being from the Underworld (the fruit), a transgression of the governing principles of the world. That woman, Hisako Yao (Yao Bikuni), is cursed with eternal life by eating Datatsushi, and at the same time complete immortality is granted to her descendants, the Kajiros, a curse that even after the body rots and turns into a mass of flesh they will continue to live. In order to lift the curse on her descendants, Hisako begs for forgiveness by offering up one of her own children to the being from the Underworld in place of the flesh and blood she stole as repentance. However, since the bindings of the curse's laws of cause and effect also apply to Hisako herself, she simply operates with the motive of continuing the ritual, so that someday it will open the gates to paradise and the people will be saved.
The reason the entire village is cursed is because the village itself is a blood relative of Hisako. Amidst them are the Kajiros, her direct descendants who are imbued deeply with the curse, and experience eternal suffering over the course even of centuries. With a frequency of once every 20 to 30 years, a younger of a pair of sisters receives the Sign (she menstruates for the first time, becoming qualified to become a bride) and is offered up as a sacrifice in the ritual, but when it fails Datatsushi appears from the Underworld and malfunctions. As Datatsushi returns from the real world to the Underworld at this time, he takes "surplus" in place of the sacrifice he came for (this "surplus" appears to be lost in the real world in the form of an area covered by a landslide), and over the generations these pile up into layers in the Other World.
The Other World is a dimension in which time and space are chaotic, and since each time the ritual fails new areas and people are sent there the top level renews itself steadily.
The red water that covers the Other World is the blood of Datatsushi, flowing incessantly from the Mirror Pool that connects to the Underworld. As a part of the Underworld's existence, those who take in the red water (in Yomotsu Hegui) are no longer able to return to leave the Underworld and return to the real world. Beings of the real world who take in the water transform in order to obtain "eternal life", eventually passing through the red sea and travelling to the Underworld. However, the blood of the Kajiros forbids them from making this journey.
Once every six hours in the Other World, for a total of four times per day, a siren sounds from the other side of the red sea. This sound signals those who have made preparations to get closer to the beings of the Underworld towards the sea, and anyone who leaves for the sea once is no longer human. Those who have not adequately prepared themselves return in umi-gaeri, staying in the Other World until the next preparations are made. Also, there are people in the Other World who have become aware of this set-up and resist, remaining as half-Shibito to stay human.
Oroboros - Spatial Loop
Since the Other World is at the border between the real world and the Underworld, it recalls the first curse, the events of when the dimensions first warped. The Other World does not operate on a clear timeframe, influenced by all of the events that occur across its layers, so sometimes places that were absorbed from the real world several hundred years ago will appear suddenly. The Other World is constructed of multiple layers and each of the them, taken from various time periods, accumulate as in a pile, visible on the surface.