Oyashiro-sama (オヤシロさま) is the guardian deity of Hinamizawa. Although Oyashiro-sama is best known for his Curse, it is later revealed that Oyashiro-sama is not actually a vengeful god, but a purple-haired goddess, who would never curse people and is horrified that her name is being used in connection to such terrible things.
The Legend of Oyashiro-sama
Over the course of the arcs, different legends and explanations arise. According to the main legend, when Onigafuchi Village was attacked by demons which arose from the Onigafuchi Swamp, Oyashiro-sama descended from the heavens to stop them and ordered them to leave the village. Since the demons had been banished from hell they had nowhere to go. The villagers took pity on the demons, and offered to let the demons live with them. Oyashiro-sama was pleased, and granted the demons human form so that they and the humans could coexist. Oyashiro-sama also stayed on earth to watch over them. Because of trying to bring about the unity between humans and demons, some also laud Oyashiro-sama as a god of love, marriage and matchmaking; as mentioned by Kimiyoshi Natsumi in Someutsushi-hen.
The arcs never give the complete story. In reality, Hanyū was one of the "demons." Exactly what that means never receives a detailed explanation, and even Rika laments that she can't understand no matter how many times Hanyū tries to explain her origins.
The Nintendo DS-exclusive arc Kotohogushi-hen reveals the details of Hanyū's past which throws light on the legend of Oyashiro-sama. The arc explains how Hanyū and her clan arrive at Hinamizawa during their search for a residence and discover that the village presents ideal living conditions to their population. In order to communicate her people's wishes to the human residents of the village, Hanyū adapts a human form.
Hanyū however could not hide her horns and this earned her discrimination from the humans. Furude Riku, the heir to the Furude family falls in love with Hanyū and they marry. Their child Furude Ōka is the first in a long line of humans to carry demon blood and the ancestor of Furude Rika.
To protect the villagers from demons, Hanyū had to sacrifice her husband and lost her ability to appear in front of humans. Nobody was able to see her except for her daughter. To combat a strange disease that was spreading throughout the country, Hanyū creates medicine and gives it to her daughter. However, powerful men desired the medicine for their political purposes, and her daughter was captured. Driven by fear for the safety of her daughter and bitterness from the betrayal of the very people she saved with the medicine, she became corrupted as a demon and massacred the villagers.
Hanyū's daughter Ōka, knowing she had become corrupted with evil thoughts, battled Hanyū and successfully stabbed her with a sword. This brought Hanyū back to her senses; however, knowing she would one day turn evil again, she begged Ōka to kill her using her own sword, the only weapon capable of killing a demon. Ōka is distraught, yet she obeys her mother's wishes and slays her, in an event that is later described as the first Watanagashi ceremony.
After Hanyū's death, the villagers revered Ōka for driving away the "demon" and thus began the Legend of Oyashiro-sama. The bitterness and fear Hanyū developed drives her away from any interaction with the village beyond observation until the birth of Rika. In this effective absence, in the villagers' beliefs and legends, Oyashiro-sama becomes a vengeful and severe male deity the villagers fear more than worship, a transformation that upsets Hanyū.
In Kamikashimashi-hen, it is suggested that Hanyū is an entity from outer space; much like Une; who tried to seek residence on earth along with her brethren by cohabiting with human beings as a symbiotic parasite. Hanyū was however rebuffed by Tamurahime no Mikoto; a similar entity who resided on earth to begin with; as she viewed Hanyū as a foreign invader in her territory. This led to the beginning of their conflict which escalates and eventually leads to the events of Higurashi Outbreak. The implication of Hanyū being an extraterrestrial being is reminiscent of Takano Miyo's theory from Document No.34 that appears in Tsumihoroboshi-hen, suggesting Oyashiro-sama was an alien.
Oyashiro-sama's True Identity
From the events of Kotohogushi-hen and the Oyashiro-sama lore, it becomes clear that the term "Oyashiro-sama" refers to either of two separate individuals. One of them is the supernatural being Furude Hanyū, who devises treatment for the strange disease that plagued the villagers. This became the basis for Oyashiro-sama's curse. The other person it may refer to is Hanyū's half-human daughter, Furude Ōka who committed matricide to appease the villagers.
Legend says that the eighth generation of first-born Furude daughters in a row is the "Reincarnation of Oyashiro-sama", and this happens to be Furude Rika. Rika is the reincarnation of Furude Ōka, and according to the aforementioned legend, Ōka would be considered as Oyashiro-sama. Rika herself however usually refers to Hanyū when she talks to others about Oyashiro-sama.
Nevertheless, there is a general consensus that Oyashiro-sama is the guardian deity who imposed rules on the villagers to avoid being "cursed". These rules were created by Hanyū and later sustained by her daughter Ōka in order to control the effects of the disease that plagued the villagers and therefore, they both fit this definition for "Oyashiro-sama".
"Oyashiro-sama's Curse" is the Hinamizawa Syndrome. As the legends develop, it becomes believed to occur when the breakage of a rule angers Oyashiro-sama. Oyashiro-sama's most well-known rules are “do not leave the village,” and “do not enter the village.” There was also originally a rule about not being mistrustful, but this was a vaguer concept, and so it has been mostly forgotten by the time of the story.
Oyashiro-sama created the rules to prevent the symptoms of the syndrome developing in the villagers or its spread outside of the village. However, in modern time of the arcs, "Oyashiro-sama's Curse" more often refers to the Series of Mysterious Deaths than the Hinamizawa syndrome. Villagers believe that Oyashiro-sama takes vengeance by killing one person while the "demons" hunt down and devour another. In reality, even though villagers call it "Oyashiro-sama's Curse," most actually believe that the murders are being carried out by someone in the village rather than Oyashiro-sama.
The Meaning of 'Oyashiro-sama'
"Oyashiro" can simply mean "honorable shrine (god)" (お社) but the more accepted meaning of "Oyashiro-sama" seems to be that it means "honorable eight generations," (御八代) referring to the eight generations of first-born girls in a row in the Furude family needed for Oyashiro-sama to be "reborn."