Reign of Vengeance (AU please leave a review)

For 20 years, Korra kept her identity a secret. As the first Avatar who wages wars against other nations, a new threat to humanity offered her a second chance. But having built her entire life on vengeance, she can't stop. Even if her quest had brought death upon her friend, bloodshed to the world, and the loss of her own humanity. (mastermind, slightly manipulative Korra)