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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Reapers. A label created by the Protheans to give voice to their destruction. In the end, what they chose to call us is irrelevant. We simply are. We impose order on the chaos of organic evolution. You exist because we allow it. And you will end because we demand it. My kind transcends your very understanding, we are each a nation. Independent. Free of all weakness. We have no beginning. We have no end. We are infinite. Millions of years after your kind has been eradicated and forgotten, we will endure.