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Nightmare Fuel: Kamen Rider
Various Kamen Rider shows have an element of horror to it. Sometimes the Monster of the Week would go a bit farther then you'd expect a show that you assume was meant just to sell toys.
Kamen Rider Kuuga: All of the Grongis have self imposed rules of who or who they kill in their games. One grongi poisons its victims (mainly high school kids) by shooting a needle that gave them fatal seizures four days later after appearing to them like a ghost via super speed. Another loved backing over people in large trucks or vans.
The former Grongi, who goes by the name of Go-Jaraji-Da, also has the distinction of being one of the only Grongi to make generally kind-hearted Friend to All Children Yuusuke Godai snap out and go ballistic, resulting in one of the most terrifying scenes of Unstoppable Rage in the franchise. He savagely pounds Jaraji's head repeatedly and bloody into the pavement, rams him with his Cool Bike, drags him to a nearby lake switching to Rising Titan form halfway when Jaraji tries to use the needles on him, and then delivers several rage-fueled slashes with his sword before brutally finishing him off with Rising Calamity Titan. Beware the Nice Ones has never been more appropriate.
Kamen Rider Agito: The Unknowns who target victims based on them having psychic powers or being blood related to someone with psychic powers.
In the first episode, the first victim of the Unknown is a school boy, whose body is found stuffed into a tree. The Unknown proceeds to do this to the boy's mother and father over the course of the episode.
Another Unknown drags victims underground and buries them alive and does so to a family of four while they were on vacation.
Theres one that draws victims into dimensional wormholes and have them fall to their deaths elsewhere.
Another Unknown creates pools of water near the victim, then violently drowns them.
Still another phases its victims into solid objects and then stops with the victim half way through. Its final victims were two pre-teen boys.
Kamen Rider Ryuki: The Involuntary Battle to the Death element of the series already made it significantly edgier than its predecessors, but Masashi Sudou/Kamen Rider Scissors being eaten alive by his Contract Monster, Volcancer is probably one of the most brutal deaths to occur in the franchise to date. Depending on your level of tolerance, Mitsuru Sano/Kamen Rider Imperer's death is either this as well or a Tear Jerker.
Kamen Rider Kiva: The monsters of the week, Fangire, are the equivalent to vampires. Except instead of just draining blood, they usually cause giant energy fangs to drain victims of life to the point the humans become transparent and shatter like glass after being being jolted.
Four words: Shin Kamen Rider: Prologue. I mean my god! His jaw is ripped in half during his transformation and antennas sprout from his head. If that's not enough he is shown to instead of a finisher rip your head off. He takes the cake.
Kamen Rider Fourze: A Kamen Rider series that takes in High School. Sounds cute, right? Well, there's a Psychopathic Man Child named Hiroki Makise who's very fucked up having stalked 20 girls at the school (and has a stalker map on them), pours out so many emails, letters and voicemails to his crushes, becomes a monster that can control people and after being scorned long enough decides to drive a bus off an unfinished bridge so they can become shooting stars. This guy is the reason that Fourze has a High Octane Nightmare Fuel Page AND fill sup most of the Fridge Horror section there.
SIC's interpretations of the Kamen Riders can be this at times, due to the artistic choices. OOO resembles a brightly-colored chimera, Kabuto is teched out, alien-looking and in some cases battle-damaged, there are bits and pieces that let you expose Ichigo's insides, etc.
The manga adaptation for the original Kamen Rider series has a much more down-to-Earth—yet no less horrifying—ending: it turned out, Shocker was being funded by the Japanese government. The Kamen Rider Spirits manga plays up this aspect of Shocker, by revealing that they also dabble in arms dealing.