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 Apollo: 2/22/2015 10:42:45 AMThe flood from halo A race of hyper advanced forerunners can beat them Space zombies = scary Can infect you and maje you one of them Gravemind took over covanent stronghold Requires the equivalent of a space nuke, to kill thier food, so they starve. [spoiler]someone also said protoss, so that might be my other option. [/spoiler]