No one has posted Gyo
Looking over the thread for the best answers and common serious answers I know:
Corpse Party: Vengeful spirits, serial killer who tortures and cuts out the tongues of children and horror movie tactics ensue
School Days: Jealous ex-girlfriend goes on holiday with protagonists head, also cuts open the chest cavity of the other heroine to see if she was telling the truth about being pregnant
Boku no pico: Gay shota/trap hentai
Grisaia no Kajutsu: A stranded teacher and his students on a school-trip resort to cannibalism and sex happens too.
Malice@doll: Prostitution robots long after humanity is extinct suddenly start developing flesh and physical sensation. Some get addicted to feeling pain and pleasure which the result can only be described as a blood/gore orgy. Not a hentai series btw
Gantz: A grisly adaption of the familiar trapped/truman show. Paints human as being extremely horrible.
Gyo: Giant mechanical walkers start infecting people with a bacteria that causes people to inflate like balloons and turn a putrid shade of green. They then shove metal piping down the bodies orafices and use the bacterias gases to operate, becoming migrating flesh mountains. Not a hentai series btw
Kowaku no Toki: Starts with a bestiality scene. The lead girl by the end has been drugged with all her limbs cut off, baked into pies and sent to her lover Then she is raped though her eye socket. A hentai series