Curse of Oyashiro
The curse of Oyashiro, also known as the series of mysterious deaths or S Incidents, refers to incidents of death or disappearance that occur on the night of the Watanagashi Festival. The legend ascribed to the curse of Oyashiro is that every year, on the night of Watanagashi, one person will die and another will disappear or be "demoned away". It is widely believed in Hinamizawa that the victims of this curse are either directly or indirectly linked to the supporters of the Hinamizawa Dam Project. A number of people believe that the Sonozaki family is responsible for the incidents, the most prominent believer of this being Kuraudo Ooishi. In order to maintain appearances of power, the Sonozaki family has embraced this theory and intentionally acts suspicious in order to inspire fear of the family within Hinamizawa residents. However, the family has nothing to do with the incidents.
The Construction Site Lynching Case
In Shōwa 54 (1979), one year after the Dam Project was stopped, a fight broke out between the dam construction workers, resulting in the death of the dam construction manager. According to news articles, the victim had attacked first due to the influence of Hinamizawa Syndrome. The workers, became terrified of being labeled as murderers and came up with the idea to cut up the body into six pieces (one for each assailant to hide): the head, the torso, right arm, left arm, right leg, and left leg. This was done because the leader, who orchestrated the plan, didn't want any of the other five to report to the police. Despite the leader's efforts, one of the assailants went to the hospital to receive treatment and confessed his crime. The rest of the group turned themselves in, but the leader still remained at large to the villagers and the right arm was never found by the police. In truth, both the arm and the leader were found by the Yamainu and the leader subsequently became a live specimen for the experiments of Miyo Takano.
The Incident at Shirakawa Park
In Shōwa 55 (1980), the Houjou family visited a viewing platform atop a cliff at Shirakawa Park. Satoko Houjou, under the influence of Hinamizawa Syndrome, pushed her mother and stepfather off of the cliff, leading to the death of her stepfather and the disappearance of her mother. The mother was pronounced dead one year later. The couple's deaths were attributed to leaning on a derelict guard rail and Satoko was rehomed with her aunt and uncle, Tamae and Teppei Houjou.
The Strange Deaths of Priest and Wife
During the Watanagashi Festival of Shōwa 56 (1981), the priest of the Furude Shrine began having heart problems and eventually collapsed. He was taken to the Irie Clinic but was ultimately pronounced dead from a heart attack. On the same night, the priest's wife went missing. Her slippers and a suicide note were found in front of the Onigafuchi swamp, leading the public to believe that the priest's wife killed herself to appease Oyashiro's anger. In truth, the incident was orchestrated by the Mountain Hounds under orders from Miyo Takano. Miyo had the priest's wife killed as the wife had been attempting to withdraw her daughter, Rika Furude, from the Irie Clinic's study on Hinamizawa Syndrome. The priest was killed to maintain the appearance of one death and one disappearance every year.
The Murder of the Housewife
On June 20 of Shōwa 57 (1982), Satoshi Houjou beat his aunt, Tamae Houjou, to death with a baseball bat. Five days after the murder, Satoshi went to buy a large teddy bear for his sister's birthday and began to exhibit symptoms of the terminal phase of Hinamizawa Syndrome. Kyousuke Irie took Satoshi to the basement of the Irie Clinic for experimentation and had the Mountain Hounds manipulate information so the murder of Tamae Houjou appeared to have been done by a drug addict. The drug addict was made to confess to the murder and later died by choking on a spoon (in Higurashi no Naku Koro ni Gou, he dies by slitting his throat).
The Fifth Curse of Oyashiro
The fifth curse of Oyashiro changes depending on the arc. Among those who die on the fifth year of the curse of Oyashiro are Jirou Tomitake, Rika Furude, Mion Sonozaki, Satoko Houjou, Rena Ryuuguu, Keiichi Maebara, Teppei Houjou, Kuraudo Ooishi, and Katsuya Kumagai. Miyo Takano occasionally "dies" during this curse, however her death is faked by burning the corpse of a recently deceased woman and fudging the autopsy report.