A Game Changer, alternatively known as Knight of Cerebus, is a type of villain who appears in comedic or light-hearted media, in which villains are not meant to be menacing, but creates a stark contrast by being played completely seriously, changing the tone of the story. To clarify, when a story portrays usually bumbling, comedic or incompetent villains, or even villains who despite posing a credible threat still have comedic moments, the Game Changers are very competent, really dangerous, sometimes even frightening, and have a much bigger impact. Episodes featuring them focus on the seriousness and drama and not on comedy or humour.

For a villain qualify in this category, they must provide situations that are much more serious or threatening than any other villain that has appeared in the same story. The very appearance of one can change the tone of the series for good and/or shatter the status-quo. Game Changers are the evil counterpart of Seriously Good. They are also extremely rarely Incompetent or stupid, because such villains are very intimidating and dangerous threats. 

Good examples of this are Nekrozoth from Multiversal Legends, Scarlet Spiker of the Inkopolis Chaos Trilogy, Turbo Mecha Sonic from Super Mario Bros. Z, The Chief from The Special Strike, Geminald from Rememberence, Novisor from GameToons, Does Bad Things Guy from SuperMarioLogan, The Egg from Dream SMP, Ominous from Sonic Overload and Sonic Rectification, Black Hiver from Skellington's Revenge and Niles from SMG4.

Guidelines:

1. They are hardly if ever comedic. Of course there are very rare exceptions namely if their gimmick is that of a clown or jester in which that is simply part of their gimmick itself. This cannot distract or lighten the mood.
2. This is not simply a dark character. It has to be of an unheard level, like a cataclysmic maniac in SML or similar very lighthearted media.
3. There are very few if any of these within a story or said media, and if you have had one it is much harder to have another one later due to a couple of reasons. After one is introduced, it is harder for another villain to standout enough to truly count. The other reason is that the audience in general isn't as easily shocked and at times even expect more like them.
4. This villain type must ultimately contrast to the current setting of the story, and their very presence darken it as a result. This feels strange and is previously unheard of within the setting.