Thread:NerdWithAKeyboard/@comment-38518844-20191217052519

Overarching Antagonist: a villain who sticks around for the entire series or at least the majority of it, overlapping any story arcs. Bigger Bad: a threat/villainous presence that's more dangerous, affects more people, or is more significant than the story's current Big Bad in the setting as a whole, but isn't causing the conflict of the immediate story (and may have little to do with it at all).

Those two terms aren't the same. It may seem like that sound the same but they decivingly  