Overarching Villain

As Arc Villain is to Story Arc, Overarching Villain is to Myth Arc. This is a villain who sticks around for the entire series or at least the majority of it, overlapping any story arcs. Because of that, they usually have a big role in the Myth Arc. The villain doesn't have to stick around for the whole series, but they must be around for a significant portion of it. Most of the time, they are responsible 100% for all events of the story (usually preparing the setting for the Bigger Bad and Big Bad to appear and start their show). A good example is Sequined Sadist who created the Omniverse, the place where all events of the story of CIS Productions happens.

Sometimes, a Overarching Villain will be behind an Arc Villain or Monster of the Week if that villain is working for them or being manipulated by them. Usually, they are behind the Bigger Bad and Big Bad. This can result in the story being Hijacked by Ganon if the Overarching Villain has been introduced previously.

Not to be confused with Big Bad. The Big Bad is the villain who directly causes the bad things that the heroes are in conflict with. This is merely a villain who has a role for the entire series or at least a significant portion. Big Bad and Overarching Villain usually overlap, but not always. 

May overlap with Arch-Enemy, the hero's most personal and recurring enemy. Compare/contrast Arc Villain, a villain who is around for a single story arc. Contrast Monster of the Week, a villain who is around for one episode. If the Overarching Villain is only indirectly involved, but is a greater threat than the current Big Bad, then they are a Greater-Scope Villain (again, not all Myth Villains are GSV).