So you're watching a kids' show taking place in a Sugar Bowl with cute characters and a jolly, lighthearted tone. Then a villainous character is introduced into the plot. But given how inoffensive the work has been up to this point, surely the villain will be a goofy Harmless Villain?
Except sometimes, these works will have villains who are legitimately intimidating and frightening. Maybe they try to destroy all positive emotions, maybe they turn people into twisted shambling abominations, or maybe they're outright plotting genocide.
Point is, they're serious, threatening, and/or creepy villains in normally lighthearted fiction that are so disturbing, so terrifying, that they clash harshly with the tone of the work. Because of this type of villain's ability to alter the mood of the story they're in, this trope overlaps with The Killjoy, Knight of Cerebus (if the character causes a lasting tonal shift in a work or series), or even Complete Monster. If a series has a lot of villains like this, then it's taking a ride on the Cerebus Rollercoaster. Done well, it provides a powerful contrast. Many family-friendly and Slice of Life shows have episodes that choose to take a turn down Nightmare Road by presenting disturbing imagery, creepy characters and dark situations as a figment of some character's imagination or nightmares. This doesn't count, however, as the characters aren't real in that universe either; they merely represent the fears and the anxieties of some individuals.
A violation of Genre Consistency. Compare and contrast the Crapsaccharine World, where it's not just the villain, but the entire world that is rotten to the core while sparkly and sweet on the surface. Also compare What Do You Mean, It's for Kids? and What Do You Mean, It's Not for Kids?. A major cause of Sugar Apocalypse and Surprise Creepy. See also Vile Villain, Laughable Lackey, which this often overlaps with. See also Evil Cannot Stand Cuteness.
