In order to add more drama to a series which has been, up until that moment, lighthearted and comedic in nature, a villain (or antagonist of some sort) is introduced and is portrayed as a genuine threat, in contrast to the bumbling and comedic villains the heroes have faced before. The villian's arrival is sudden, and he is usually either a totally new character, or a character that has undergone a particularly dramatic FaceHeel Turn.
This villain's arrival is heralded by a sudden downturn in humor (although if the villain is Laughably Evil, expect it to turn into Black Comedy), showing the audience this one means business. In other words, a catalyst for a drastic change in mood toward the dark and dramatic. Some characters may take the story to darker territory without being villainous in the slightest, but this trope is reserved for evil creatures. Grim Dark shows and stories that take place in a Crapsack World are much less susceptible to this kind of influence, but even they have exceptions if this villain shifts the story from Black Comedy to being more serious. This isn't only about the character themself, it is also about their relation to the story.
A Knight of Cerebus may be a blatant Hero Killer who poses an actual threat, or even an Invincible Villain. They also tend to be the trigger of The Worf Effect against the hero. They may be the Arch-Enemy as they might have a strong personal tie to the hero or heroes of the work. They are usually distinguished from the other villains as perhaps being worse, stronger, or more personally tied to the protagonists. Sometimes this type of character can be a Complete Monster if they have no redeeming traits and do terrible things by the standards of history in an initially more lighthearted series. Often this character's arrival is a Gut Punch.
See also Dead Serious and Not-So-Harmless Villain, when a villain starts as ridiculous but, because of some changes, ends up becoming this. Compare Shoo Out the Clowns, when the darker tone coincides with decreased screentime for comedic characters.
Remember, Cerebus Syndrome is a prerequisite for this trope. The Knight of Cerebus heralds a long-term tone shift in an episodic work and cannot exist within a single installment. An increase in danger as a conflict escalates is present in almost any story with a villain, and is not automatically notable. Can overlap with Vile Villain, Saccharine Show when the setting itself contrasts heavily with the character's grimness.