What makes the flood and chief op?
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The Chief is insanely luck due to the power of plot devices. Up until 4 he also had the most advanced AI ever created by humans. The Flood have sentient Graveminds and can take control of literally and ship and use it to cook their "food". Their army also grows the more they kill. They were so powerful that the Forerunners had to do a hard reset on the galaxy to stop them.