Death's Head Rebellion

After the collapse of the First Empire of Man, a war was fought between "old style" humanity and the Sauron supermen. Humanity eventually annihilated the home world of the Saurons but the cost was high: the Empire fell into a new Dark Age in which the secret of interstellar travel was lost. Now the remnants of the Saurons after returned but did not anticipate a mutiny in their own ranks.