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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Willy Wonka, he lures in you into a factory. Figments of his imagination live there. We've seen the pleasant stuff and it was enough to kill several kids(which he planned on killing FYI). What we have not seen is the twisted and depraved part, the part that is in all our minds.