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.
His close up as he's about to stab Jack, his eyes look creepier than normal.
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. What really seals the deal is the cacophonic distortion in the "other realm" Jack ends up in, which sounds like something you'd get if you blasted really loud music out of your speakers at full volume.
The Haunted House seriously needs to be seen to be believed. The creepy visions Jack keeps seeing to some unnamed thing attacking the occupants of the house, the eerie tranquility in the fake tea room, not to mention that, unlike most villains who appear on the show who all either work for Aku or are involved with him some way, the Demonic Spirit has nothing to do with Aku. Its just... there.
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.
Jack had to rely on the power of a god to destroy them. The god himself, Ranote The very same Ra that, along with Odin and Vishnu, destroyed the evil entity that would spawn Aku, was pretty creepy as well in the fact that he did so with very little effort, the fact that he towers over Jack, and that he does nothing but stare at Jack, not even speaking when Jack asks him if he knows a way to return to his own time.
Jack: These creatures are more powerful than anything I have ever encountered...
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 Aku crashed to prehistoric Earth, he took the form of a meteor that burned nearly all of the dinosaurs down to charred skeletons, then proceeded to gobble up the few remaining survivors. The carnage is abominable.
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.
Worse than Aku is his Bad Future world he teleports Jack to. No one knows Jack and most of the people either worship Aku or are slaves. It makes one shudder to even think of living Ina Bad Future controlled by a Reality Warper master of darkness.
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 propaganda stories to children.
Also note that they didn't just attack other robots, they destroyed two other towns, one with little cat-like creatures and another with a population that bore a resemblance to The Sand People. The latter of which is notable in that Jack encounters no survivors.
"The Aku Infection". Aku spreads some evil that infects Jack and soon starts to look scarier and scarier as it goes on.
Jack: I'm getting...
(lifts up cloak to reveal Jack is looking and sounding more like Aku)
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...
In a manner of speaking. Its implied that the original Abomination was only capable of mindless destruction, while Aku is sentient.
"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.
Even earlier, Jack is traveling through the scorched countryside that was once the Lava Monsters home, implied to be in Norway. It's bleak, cold, and littered with ash and dead corpses. On top of that, there's an ominous voice constantly beckoning Jack to follow it.
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.
In an episode called "Samurai vs Ninja" it gets downright hectic and overall a bit eerie since you have two of the most deadliest warriors hunting one another in the art of Shinobi. All the while that they're both cladded for the appropriate elements. One fights in the dark while the other fights in pure blinding light. The whole fight in the very wooden tower is both a respectful mix of Nightmare Fuel and Paranoia Fuel. And to add even more fuel to an already deadly, adrenaline rushing encounter? The soundtrack throughout the whole fight! ... Horror cords and all. Hereis the fight in its entirety.
Aku's assault on Jack's home city in the first episode. He shows up looking bored, the garrison attacks him, and finally he smiles entertained as he wipes them out with the very arrows they shot at him, and then he starts attacking half-seriously.
Samurai vs Samurai is mostly a goofy episode involving Jack dealing with the Da Samurai, until the last few minutes. Suddenly, an army of Aku's assassinbots from earlier in the episode show up, but these ones Dual Wielding swords and opening the panels in their heads to reveal creepy long faces with razor-sharp teeth. That alone is scary enough, but after Jack finally defeats the entire army, they all combine into what can only be described as the mechanical equivalent of an Eldritch Abomination. Thankfully, it goes down easier than the army did, but it's still freaky to watch.
The creatures that Jack fights in "Chicken Jack" ranged from downright creepy to outrageously grotesque. And this was a lighter episode!