History NightmareFuel / Ben10

** [[spoiler:And when he finally get defeated, it's done at the cost of Kevin's sanity, causing to turn evil again and become even more AxeCrazy than he already was in his youth.]]

* Zombozo shows up again, and while his look isn't as creepy as in the original show, he compensates by acting in a completely AxeCrazy, Joker-esque way; He kidnaps Ben's mother and places her in a deadly trap so she would die gruesomely while Gwen was watching, was implied to have buried someone at the beginning of the episode, and attempted to shoot at Ben with acid. Worse is how Gwen punishes him for attacking their loved ones- it scares him enough to return him to his stupor he was in after Ghostfreak spooked him.

* Captain Nemesis wasn't much more than a Silver age superhero parody turned evil in his first appearance; yet, you won't laugh at him anymore in his next appearance, where he gets to be the first character of the show to ''succeed'' in killing innocent human beings. Several times. With at least one body shown (though from behind). And this is supposed to be a kid's show?
** Well, when you undergo plastic surgery via the hand of someone best described as an Igor {{Expy}}, there's bound to be a line that gets crossed... namely, Gwen catching the leftover scent of the man's ''blood'' in the operating room.

--->'''Highbreed''': It sure is. I told you to bring me the crystal, and you didn't. Maybe you misheard. Maybe ''you'' need to have ''your'' ears cleaned... (loud hissing noises come from the acid dripping out the ear cleaner [[GoryDisretionShot offscreen]] as the episode ends)

* Ghostfreak's return had some creepy moments, especially the way he takes over Vilgax's planet.

* Captain Nemesis wasn't much more than a Silver age superhero parody turned evil in his first appearance; yet, you won't laugh at him anymore in his next appearance, where he gets to be the first character of the show to ''succeed'' in killing innocent human beings. Several times. And this is supposed to be a kid's show?

* Simian is accidentally responsible for infecting his planet with Xenocytes- y'know, the nasty parasites who mutate anything they feed off of into a ''DNAlien?'' Just think of the DNAliens you've seen up until now, only as monkeys with four arms. Surreal and scary, huh?

* Reirassic III's debut- he had his arm ''severed'' by a desert parasite, complete with indigo AlienBlood dripping out. Luckily, Ben infused Swampfire's regenerative plant cells into the Highbreed's detached arm and reconnected it to his body- but even so, his arm became a hybrid of flesh and chlorophyll, turning it completely green, along with all the neighboring arteries and veins hooked into it where the chlorophyll eventually melded with the flesh. CreepyCool, but also nasty.

* The Hightbreeds could be creepy on occasions, especially the one who was about to punish Simian at the end of his debut episode:

--->'''Hightbreed''': It sure is. I told you to bring me the crystal, and you didn't. Maybe you misheard. Maybe your ears needs to be cleaned... (end of the episode)

* Zombozo shows up again, and while his look isn't as creepy as in the original show, he compensates by acting in a completely AxeCrazy, Joker-esque way; He kidnaps Ben's mother and places her in a deadly trap so she would die gruesomely while Gwen was watching, was implied to have buried someone at the beginning of the episode, and attempted to shoot at Ben with acid.

* Captain Nemesis wasn't much more than a Silver age superhero parody turned evil in his first appearance; yet, you won't laught at him anymore in his next appearance, where he gets to be the first character of the show to ''succeed'' in killing innocent human beings. Several times. And this is supposed to be a kid show ?

Let's face it, a [[{{Ben10}} meta]][[Ben10AlienForce -serie]] [[Ben10UltimateAlien about]] [[Ben10Omniverse Aliens]] ''has'' to involve some NightmareFuel.

* Depending on the alien Ben is turning into, some transformation sequences can involve a large amont of BodyHorror:

** Ripjaw show's Ben's mouth distording itself and growing monstrous teeth;
** Fourarm, Xlr8 and Wildmutt in the original series have Ben's muscle growing in a quite disturbing way, making it look like his nerves are becoming apparent;
** Ghostfreak has Ben's head turning into a skull during the first part of the transformation; it's even worst in Alien Force, where we get to see his spin distording, his claws emerging in a Wolverine-like way from his hands, and his skull making a 180° turn;
** Goop has Ben litterally ''melting''

* [[BigBad Vilgax]] can be quite terrifying from some angle; his voice, provided by SteveBlum, is creepy by himself, especially when he is mostly seen in the shadows in the first episodes. When he finally ends his treatment and goes after Ben of his own, he proves to be unstoppable, defeating most of Ben's alien forms by pure strenght. And add to that he was ready to cut off Ben's arm to get the Omnitrix.
** Even worst in "Gwen 10", where it's made clear he doesn't ''need'' to cut Ben's (or in this case Gwen's) arm to get the Omnitrix. He just rathers doing it that way because it's more fun for him...
** And then the movie ''Secret of the Omnitrix'' reveals he destroyed a planet just for the sake of testing his newly aquired toy.

* [[MonsterClown Zombozo]]; not only is his look disturbing, with a zombie-like face and a longue, inhuman tongue, but the guy feeds on people happyness. Not scary so far ? Well just see what is left when he has sucked most of it...

* Anything related to Ghostfreak; from his creepy transformation sequence, to the nightmares he made Ben go through when he was attempting to escape, to his true form, to his scheme in season 3 covering Earth with darkness and turning everyone on the planet into monstrous mutants.

* In case Zombozo wasn't creepy enough in the original show and Ultimate Alien, Omniverse give him a more zombie-like appearance, with visible ribs, an apparently missing eyes and decayed flesh in some places.

* The episode introducing Darkstar was downright creepy; it involves schoolgirls being turned into zombies slaves by the main villain, who uses them to get energy to feed him. The way he seduces them is subtily reminding of rape in some scene. Plus, this was one of the few episodes in this season where the villain actually was a CompleteMonster.

* Aggregor; the guy is probably one of the most competent villains in the whole serie, maybe the whole franchise; He did nothing but win for all his story arc, usually using the protagonists as pawn. And he was so much of a CompleteMonster that he was ready to absorb (which might as well be kill) five aliens in order to get their powers. Even worst, his final plan was to [[spoiler:[[PoweredByAForsakenChild absorb a Celestiansapien child]]]]. This guy was in many way similar to Kevin 11 in the Ben 10000 future where he didn't reform.

* Depending on the alien Ben is turning into, some transformation sequences can really freak you out:

Let's face it, a [[{{Ben10}} meta]][[Ben10AlienForce -serie]] [[Ben10UltimateAlien about]] [[Ben10Omniverse Aliens]] ''have'' to involve some NightmareFuel.

* Zombozo shows up again, and while his look isn't as creepy as in the original show, he compensates by acting in a completely AxeCrazy, Joker-esque way;

* [[EldritchAbomination Diagon]]; his minions, the Lubracas, are weird demonical creatures with the abilitie to brainwash humans merely through physical contact. And they are just bugs compared to Diagon himself.

* Aggregor; the guy is probably one of the most competent villains in the whole serie, maybe the whole franchise; He did nothing but win for all his story arc, usually using the protagonists as pawn. And he was so much of a CompleteMonster that he was ready to absorb (which might as well be kill) five aliens in order to get their powers. Even worst, his final plan was to [[spoiler:[[PoweredByAForsakenChild absorb a Celestiansapien child]]]].

* [[EldritchAbomination Diagon]]

* Aggregor; the guy is probably one of the most competent villains in the whole serie, maybe the whole franchise;

* Diagon

* [[MonsterClown Zombozo]];

* The episode introducing Darkstar was downright creepy; it involves girls being turned into zombies slaves by the main villain, who uses them to get energy to feed him.

* Zombozo shows up again

* The [[{{Mooks}} DNAlien]] are creepy on their own to an extent, but what can really be scary is the way they are created: the Hightbreeds use Face-Hugger-like creatures to infect humans, turning them gradually into DNAliens. Which would mean each time the heroes kill [[{{Mooks}} DNAliens]], they are actually killing human beings (and it ''does'' happen)

* The DNAlien are creepy on their own to an extent, but what can really be scary is the way they are created: the Hightbreeds use Face-Hugger-like creatures to infect humans, turning them gradually into DNAliens. Which would mean each time the heroes kill DNAliens, they are actually killing human beings (and it ''does'' happen)