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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Edited by EnquiringNewt: 5/26/2015 6:19:53 PMNecrons from Warhammer 40k. A race of beings imprisoned in mechanical bodies that are bent on restoring their former place as the rulers of the stars. The only thing that has ever stopped them is themselves.