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Nightmare Fuel: YuYu Hakusho
Yu Yu Hakusho, as much as it is a shounen series, let us remember that Yoshihiro Togashi was inspired by H. R. Giger the artist who designed the titular Xenomorph Aliens, and that the 90's was a grand time for a genre best described as 90's Horror Anime. The series' debut came out at a time when lighting helped give 90's anime a gloomy and airy effect for atmospheres, and considering Yu Yu Hakusho's themes of the supernatural, eldtrich, and the infernal, the series also does not skimp out on its genre.
Chapter Black. The fact that all of mankind's worst moments, from acts of genocide, to slavery, to even war and destruction of all sorts, unadulterated and uncut, were put on tape, and you could watch real life cruelties and evils committed from all throughout time.
Reading into Sensui's Mind. Murota goes insane over the fact that Sensui chants nothing but how much he would like to see piles of bodies, from adults, to children, to the elderly, piled in graves, and butchered in any feasible and possible manner, and gets more and more ecstatic about killing EVERY human.
The Anime was darker and less discreet about showing us what on earth could happen should Yusuke and the crew die or fail on their mission, and what on earth they had at stake. And they did NOT skimp out on the details.
The Makai Insect infestation.
The result of the opening of the entrance to the Demon World. GOOD FUCKING LORDS.
The Sinning Tree, and Elder Toguro's fate. He doesn't die. He never will. An eternal life under a nightmare and your worst fears to haunt you forever. Give that a thought.
Quite fitting in that pretty much everything about Elder Toguro is Nightmare Fuel actually, up to and including his death. His Lovecraftian Superpower, creepy head in a fish tank, killing Gourmet from the inside, he probably also ate Murota as Gourmet, even his personality could be considered Nightmare Fuel.
Younger Toguro going 100%, it starts off with a gaunt body, compressing the power, it bursts into purple energy that later shrouds the arena in darkness. After which, Toguro greatly enlarges his body (which sounds very painful) through bursting muscles to unnatural extremes, we see a close up of his new muscle skin as well as which appears to be breathing just before the transformation completes.
During his No-Holds-Barred Beatdown at the hands of Toguro, he tries to escape into his dreams where he sees himself and Keiko as children. Toguro appears as an enlarged version of himself to grab her, saying its too early to escape into his dreams.
During the Sensui fight, there's one pretty unsettling Mood Whiplash Sensui shoots Yusuke, and soon takes a completely different persona. That being a sadistic gunman called Kazuya who gives Yusuke a No-Holds-Barred Beatdown by gunning him down and grinding his wounds, This wouldn't be so bad, if we didn't learn that he was an admitted baby killer and loves the sound of screaming way too much. Thankfully, Koenma shows up before Kazuya decided to blast through Yusuke's skull. Quite a twist from a suave manipulator.
Hiei's idea of anice birthday gift for Mukuro. Actually, this is almost funny in a sick, sick, sick way, seeing as that's the guy who used her as a sex slave, but still...
Mukuro was "fitted" to be used as a sex slave when she was just a NEWBORN BABY. In other words, she was raped since age 0. Brrrr.
While Kurama has a fancy rose whip, he's capable of summoning man eating plants with acidic saliva, blood sucking plants that literally either burst from the victim's body or drain them dry, and here's the kicker, a plant that constantly drains your life while giving you hallucinations of your worst fears, potentially a living hell for any impossible to kill psychopaths. Yes he is a nice guy.