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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Im saying timelords 1. Time travel and invented black holes (don't ask me how, they did, somehow.) 2. Bigger on the inside technology 3. Can regenerate