The Talon of Horus pg. 398

And still they died with every swing. Gunfire shattered the primarch's black ceramite and blew fist-sized chunks out of his flesh from his bones. Pain threading his aura, yet Horus fought on. I threw energy at him. I threw lightning. I threw panic and hatred and anger in a seething bolt of mutagenic warpfire. It burst what remained of his force field in a whiplash of air pressure, and boiled the skin and hair from his head. Nothing more. I was still too weak, and he was far, far too strong.