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 main legend, when Onigafuchi Village was attacked by demons from 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.The arcs never give the complete story. In reality, Hanyū was one of the "demons." Exactly what that means never receives full explanation, and even Rika laments that no matter how many times Hanyū tries to explain what she is and where she came from, Rika can never grasp it. Most of the details of what actually happened come from the later arc Kotohogushi-hen. Hanyū took on human form; however, she could never remove her horns. The humans contracted a disease in someway connected with the villagers, later known as the Hinamizawa Syndrome, which Hanyū and her people bring under control. To prevent the spread of the Hinamizawa Syndrome, villagers were forbidden to leave the area while strangers are discouraged to enter. Hanyū married the Furude heir, and their child is a direct ancestor to Rika. Nevertheless, conflicts remained between the humans who cannot accept the visitors. Ultimately, Hanyū sacrifices herself to save the village and becomes its goddess: Oyashiro-sama. The bitterness and fear she 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.
"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 vengence 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."