User blog comment:James12708/Reapers VS. Forerunners VS. Angels from Darksiders VS. Humanity from Halo VS. Council and humanity from Mass Effect VS. Imperium of Man/@comment-1440333-20130206000208/@comment-1440333-20130207065949

One word-- Psykers.

The Imperium unleash the full powers of the Warp on the Forerunners and they die screaming.