It's the antagonist who's basically the antagonist of the greater scope of the story. As opposed to antagonist who're just an antagonist of a particular story arch. The Overarching Antagonist could also be the one behind everything, in other words, they could be the one who's ultimately responsible for certain conflicts even if they aren't the primary antagonist for them.

Thank you!

You're welcome :)

I thought that an overarching antagonist is an antagonist who is indirectly responsible for the plot of their respective media happening.