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Nightmare Fuel: Samurai Jack
If you watch the whole series without at least one squeak of terror, then you have more gumption than Jack himself. Some episodes were entirely built around Nightmare Fuel, like the episodes featuring the trip through a zombie graveyard, the Minions of Set from Jack In Egypt, and the haunted house. Even the episodes not revolving solely around Nightmare Fuel have scenes that are simply traumatic, the frequent robot carnage that the censors let pass in every episode occasionally goes way beyond the borders of "not too bad;" once you reach dismembered robots impaled on spikes, things have gotten bad.
Speaking of the graveyard episode there is a little bit of Fridge Horror that makes Jack's 'battle', and by battle I mean simply trying to survive, against Aku even more frightening. The form that Aku takes to finally overwhelm Jack is a huge black mass similar to the abomination he was a part of. Just a horrifying reminder that even though he's gained sentience and likes to toy with people using different forms, he can become a simple unnatural force if needs be.
Hey, I've got an idea, let's talk more about the haunted house! Jack follows a little girl to an old, decrepit house. He receives several nightmarish visions about what happened to the various family members that used to live there, all of them appearing to have been killed by some faceless horror. Not even Aku, just some creature with a sadistic streak. And that's only the first half.
And the Minions of Set episode, its only against three beings. But these are creatures that Jack himself cannot stop. Any cut he makes the limbs grow back instantly and the minions continue the chase. Jack is forced to run and go on the defensive for rest of the episode as he tries to locate artifact pieces that will help defeat the minions. Nearly getting killed in the process. To say it's tense is an understatement.
Episode 15, "Jack's Tales", has Jack encounter a family of metal-eating robots. The robots, in a fit of hunger, rip each other to shreds, and the insides of the robots look just graphically enough like human flesh to creep you out.
Aku himself suits this. Especially in his first appearance in the pilot.
The two-parter explaining his origin adds to his terror. Apparently, he started out as this freaky monster inside the earth that would send large, deep cracks through the ground. The cracks were filled with him as he took the form of a tar-like substance, and he would then absorb living creatures to get stronger. At one point, a man calls out for his dog, goes to the back of his house, and sees one of the cracks filled with Aku-goop, and half the dog's leash sticking out. Doubles as a Tear Jerker when we hear the man's anguished and horrified screams.
When Jack's father goes to stop Aku, charging into the forest of black, spiky trees with an army of samurai in tow. As they get closer and closer more trees burst up from the ground, and we hear samurai start screaming as their horses (now riderless) run by Jack's father. And then, after Jack's dad wakes Aku, Aku ties him up, slips into the ground, then pops back up by his kingdom and attacks it. Coupled with the guy's horrified face and the Big "NO!" that follows, imagine how the people in the city felt.
Even one of the most hilarious episodes where he's involved merely paints him horrific in a different light. He rounds up kids from within a capital city, with no reason given to them or their parents. The room they're led to is large, red and spiky as always, but has monitors of adorable, yet...off animals, which eventually switch off. The camera focuses on a large red throne in the room. And Aku just appears. No fanfare, no flashy effects, no booming monologue. He's just there. This is how he reads propoganda stories to children.
In the second episode there are silhouettes of crucified, intellectual dogs, and it's implied that they are dead; Jack's reaction upon seeing it is entirely justified. Aku has been doing this since Jack's original time period to multiple races throughout multiple galaxies. For thousands of years. That's horrifying. It makes you wonder if the universe was better off with the original abomination...
"Jack And The Zombies". Some of those zombies were just plain freaky, like the long-armed zombie and the one that possesses Jack's sword. And when he falls into the crypt... that wail.
Aku: Oh, Samurai? Where are you, Samurai? You can run, but you cannot hide, 'cause I can smell your blood!
"Jack And The Lava Monster". As Jack comes closer and closer to reaching the end of an increasingly lethal series of traps placed by a cursed warrior, you see dozens of skeletons strewn about his labyrinth of death, killed in brutally horrific fashion when they each made a lethal mistake on the way in.
Worse still, after Jack breaks the curse on the warrior, he returns to human form- but the entire time he was locked away in that demonic prison, he was separated and left untouched from the natural flow of time. With the curse no longer keeping him filled with vitality, the effects of millenia wearing down on his physical body are finally unmasked- he reverts into a frail old man in an instant and dies. Fortunately, he's carried off to the afterlife paradise of Valhalla to live in eternal bliss.
Demongo, dear God. Not only is he an Eldritch Abomination, he steals the souls and keeps the skulls of all the warriors he's slain. Skulls that he wears on his chest. Skulls that he can toss out to summon the ghosts of the fallen warriors to do his bidding, all of which have creepy blue mind-control eyes.