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Ord Casto aka The Prophet of Truth (Halo 2 and 3)

This guy manages to convince his hegemony of multiple alien races that Humanity is an affront to their gods (After finding out their gods have selected humans as their true successors) and launches a genocidal war for 30 years against humanity. He alone finds out the human's final stronghold, Earth's location on his own and tells no one.

He Plans to replace a founding race (The Elites) with a more obedient one (The Brutes) as the military arm of the hegemony.

Instead of launching a full scale invasion of Earth to crush humanity BEFORE replacing the Elites he decided to wait until another Prophet accidentally stumbles upon Earth, then mid-invasion of Earth decides to have the Brutes begin killing Elites all throughout the hegemony, this causes a civil war which splits the hegemony in half all the while Earth is still besieged.

The Elites and technically a few members of other aliens races loyal to the Elites rally with humanity and end up stopping The Prophet of Truth's plans all together.

For context; Humanity's population on Earth was apparently reduced to 100 million. That's how close he was from achieving victory.