This includes fantasy and sci-fi.
My choice is the Combine.
A multi-universal species from half-life so absolutely immense and powerful that they reverse-engineered earth's technology, upgraded far beyond human ability, brainwashed some humans and had them invade themselves. They did this and stomped all resistance within 7 hours. This was done by a backwater lesser combine city mind you.
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Precursors. 100 billion year old manipulators of matter. They can create universes ad parallel dimensions at will. Their Star Roads make the HALO array look like a firework. They can't die, only be reborn into a different form. And they are also the Flood.