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. They got pissed off when they realized they'd been wiped out, so they created a revenge plan that would occur over millions of years and turned themselves into an all consuming parasite; just to get back at some people that ticked them off. Not to mention they alter reality and create solar systems and galaxies for shits and giggles.