Nightmare Fuel / Zatch Bell!

A Broken Smile from manipulation, courtesy of Zofis.
  • Episode 121 of Zatch Bell!. Tia (or Tio) gets pissed off at a mamodo named Momon, who is known to be a pervert because of the use of his evasion-themed powers. As she gets angrier from his taunts, her powers grow and her face becomes monsterous. Soon, she gains a new spell called Chajiru Saifodon, an attack in which the power of the spell depends on her anger. At first, the spell will take an appearance of a sword with goddess-like figure on top and a mirror in the middle that displays negative experiences she dealt with at the time. As said experiences are displayed, the goddess' face will grow more distorted over the course, eventually showing demonic traits and starts screeching. It probably didn't help that her close friends were afraid. The spell has to be seen to be believed. Her pupil-less Slasher Smile she made when Momon slowed her down deserves more points of scary.
  • Clear Note is a unnaturally human like Mamodo as well as the final Big Bad of the series, whose power is annihilate anything that comes in his radius, well as reduce arms to withered stumps due to the pressure As Brago can testify
  • Any time Kolulu transforms into Zeruk, complete with a deeper voice, muscular appearance, and a never-ending thirst for blood.
    • Also the fact that apparently good Mamodos who don't want to fight are given alternate personalities that do shows just how messed up this whole competition is...
  • The thousand-year demons. They were sealed in stone, completely conscious, for an entire millennium. Made worse when you remember that the demons in this competition are children, sometimes as young as six years old.
  • In episode 36, Zatch has a nightmare about Naomi. Needless to say it's creepy as all hell.
  • Zofis' manipulation of Koko. Koko started to gain a degree of insanity and a cruel personality because of his mind-controlling abilities. One of the first things she does under his influence was to burn down her home village as revenge for the cruelty she endured. All with a Broken Smile on her face.
  • The very first time we see a Mamodo's book burn. Sure, we later learn it's not nearly as bad as it looks, but seeing a childlike entity desperately trying to put out their book as they apparently fade from existence is more than a little unsettling before we know what's actually going on.
  • Zeon's own Slasher Smile in the manga where he talks about wanting to see everyone's Despair Event Horizon when he tries to destroy Japan is so evil and nightmare-inducing that it makes Tia's abovementioned expressions look charming by comparison.