Rinkan Club

A 4-episode OVA released between 2011 and 2014 that focuses on a cult in the fictional town of Rinin, Japan. That cult's main ceremony involves the gang raping of women, designated "shrine maidens". Said ceremony is conducted by the Rinkan Club, which can be translated as either "The Rape Club" or the "The Gangrape Club".

A dark ceremony existed for thousands of years in the old town of Rinin, handed down from generation to generation. The name of the ceremony, in particular, is in the oldest of records. That ceremony, enacted to protect the region from disease and crop failure from a curse from the region's guardian deity, Iwanaga-Hime, occurs in the evenings between the new moon and full moon.

A maiden is disguised as Konohana Sakuyahime, another goddess, and gangraped to appease Hime Iwanaga's anger.

In regard to the maiden, there must be a special kind of woman participating in the gang rape, in other words, that woman must be selected by a blood relative, husband, lover, or themselves. After the gang rape, if a pregnancy occurs, Hime Iwanaga's anger is abated, and a good harvest and safety upon the region will be promised. However, if a pregnancy does not occur for the sake of suppressing Hime Iwanaga's anger, the maiden is supposed to be covered with semen, and floated down the river alive, as a sacrifice.

After the Taishou era, the ceremony itself had vanished into the dark history, but kept occurring at Rinin High School; the cruel, improper traditions being inherited over into a secret society through actions carried out by the Gang Rape Club. For generations, the president of the student council of the High School has served as the head of the Gang Rape Club, not allowing even one day to pass uneventfully, submerging numerous women to the dark, suffocating pleasure. The OVA follows Suzuko Yamazaki as she tries to put an end to the ritual.

The OVA was Pixy-Soft's final work before animation work for Lilith-Soft, the people who made Taimanin Asagi, were transferred over to ZIZ entertainment.

Tropes used in Rinkan Club include:

Adaptational Villainy: The Club is shown using coercion to get women to be shrine maidens, something the protagonist in the original game openly refuses to do because it is against their rules. The supernatural elements are also much less explicit to the point of being ambiguous, making the club's members obedience to the orders look like they just follow strict rules rather than trying to really calm supernatural entities.
Absurdly Youthful Mother: Aiko Katsuagi. The woman looks great for someone with an adult daughter. Doubles as an "Absurdly Youthful Grandmother" after her daughters, Shiori and Maki become pregnant at the end of the OVA.
A Date with Rosie Palms: A student raping Aiko in Episode 3 confessed he masturbated to her before.
Adaptation Distillation: The OVA is much more condensed than the visual novel, and actually remove characters like the nurse Natsuko Hayasaka, and the students Izumi Shinohara and Chiaki Yukimi.
Babies Ever After: The 4 women that were sentenced to be "shrine maidens/priestess for life" all ended up being impregnated by the Rinkan Club.
Big Bad: Motoko Tamura, the leader of the Rinkan Club. Although one can argue that Suzuko Yamazaki eventually takes over the position.
Brother-Sister Incest: Poor Tomomi. Her own brother lusts for her and sacrifices the girl towards the raping club so he could have sex with her.
Cult: The titular Rinkan Club or Gang Rape Club.
Downer Ending: The OVA ends with Suzuko becoming the new leader of the Rinkan Club and giving her Arch Enemy Motoko the Sexual Karma she deserves. Suzuko was able to abolish cult's old system, ensuring that most of the women of Rinin will no longer be victimized by the Rinkan Club's practices. However, to keep the customs going to satisfy her new followers, Suzuko decide that a few women and their future daughters will be "shrine maidens/priestess for life". Naturally, she chooses Motoko as one of the women who would spend the rest of her life being gang raped by the cultists. Surprisingly, she also chooses to sentence the innocent Maki Katsuragi, Shiori Nakamura, and Aiko Katsuagi to the same fate on the grounds that they are so mind-broken, it would be an Unishment to them. However, considering the tradition now becomes hereditary and it's pretty likely the women will have multiple daughters and their daughters will have multiple daughters thanks to constant rape, Suzuko only changed the way evil is done, but is still evil.
Full-Circle Revolution: Suzuko spends most of the OVA trying to put an end to the Rinkan Club. She was eventually able to wrestle leadership away from Motoko and took over the cult herself. However, Suzuko decides to sentence Motoko Tamura, Maki Katsuragi, Shiori Nakamura, and Aiko Katsuagi to be "shrine maidens/priestess for life". Which mean the Rinkan Club's ritual will continue, but using these 4 women and their future daughters instead of assigning a different woman from Rinin each month.
Gecko Ending: The OVA has its own original ending that does not happen in the original visual novel. The OVA ending has Suzuko taking over the cult and condemning Motoko, Maki, Shiori, and Aiko to "shrine maidens for life".
Hospital Hottie: Aiko Katsuragi, who is the only doctor in Rinin.
Hot Teacher: Both Suzuko Yamazaki and Shiori Nakamura.
Human Sacrifice: Basically, what "shrine maidens/priestess" are, but instead of being killed, they are repeatedly gang raped by the Rinkan Club for about a month. The shrine maiden is supposed to represent the beautiful goddess, Konohana Sakuyahime. According to the cult's version of the legend, in order to spare the region from Iwanaga-hime( the village's guardian deity and Konohana Sakuya-hime's sister who is envious of her sibling' beauty) from cursing the land, a shrine maiden/priestess is needed every lunar cycle between the new moon and full moon for Iwanaga-hime to mistake as her sister. A shrine maiden can only be appointed by a blood relative, a lover, or said person themselves by signing a paper robe.
Loophole Abuse: What can be defined as rape? The Rape Club uses it to torture Suzuko by making her use a vibrator and write degrading words in her body and forcing her to keep the paint on and use the vibrator even in class.
Love Makes You Evil: Downplayed. Motoko become the leader of the Rinkan Club so that she and her eventual girlfriend, Maki, will not be designated as "shrine maidens".
Making Love in All the Wrong Places: The Rinkan Club uses the school as they please to torture women. Infirmary, on the outside, bathroom, no place is safe, only the women who haven't been selected as shrine maidens. Yet.
Male Gaze: Even when characters are clothed the camera tends to focus on their legs, glutes and breasts.
Maybe Magic, Maybe Mundane: Motoko follows the principles of the cult to the letter to make Suzuko become a sacrifice (by making Motoko's long-distance boyfriend unknowingly write her name on the paper used for offering), despite of the effort it takes. This implies despite most cultists not seeming to believe in the ritual, they do it in the proper way, implying they're not only replicating it for fun. Whether the legend is real or not is never made clear, however.
Mythology: The story of Konohana Sakuyahime and her older sister Iwanaga-hime from the cult's legion is actual real-life folklore from Japanese mythology, just with the gang rape elements removed.
Police Are Useless: It's implied by Motoko in the 1st episode that the police force in Rinin is being controlled by the club, hence why Suzuko can't seem to get anyone to help her stop the cultists.
Schoolgirl Lesbians: Motoko Tamura and Maki Katsuragi, the two eventually become a couple.
Token Wholesome: Tomomi Yamanaka and Maki Katsuragi, at least until the Rinkan Club gets their hands on them.
Town with a Dark Secret: Rinin. As the entire village is being controlled by the Rinkan Club, and the authority's figures allows the cult to continue their depraved rituals.
Unishment: Suzuko believes her "shrine maidens for life" sentence she gave Motoko, Maki, Shiori, and Aiko would be this to them as they have become mind-broken to the point that they would actually enjoy spending the rest of their lives being gang-raped by the cultists.