the Night Haunter Konrad Curze

That was him as primarch of Night Lords. They were created to make threats of thorough and sadistic annihilation believable by being thoroughly sadistic annihilators.

First to actually notice that Big E only wanted to control and subjugate humanity. Not because of his prophetic abilities, but because of his thoroughly bleak understanding of human nature. To never suffer consequences of your mistakes means you will repeat them again and again with even bigger scale.