Codex: Chaos Space Marines (2007), p14 — The Attack on Earth

With the initial attack stalled, Horus opted to bring forth his entire arsenal of weapons and warriors to overthrow the Emperor. For over a month, the gigantic cannons of the rebel army pounded the walls, foremost amongst them the siege weapons of the Legio Mortis Titan Legion, which had turned to Chaos in the earliest days of the Heresy. Eventually, after heavy bombardment, part of the curtain wall came crashing down and the Traitor Legions hurled themselves at the breach to pour into the inner palace.

Along banner-lined corridors, through mile-long galleries, the loyalists and rebels tore at each other. At the heart of the fighting stood the Primarchs. Rogal Dorn of the Imperial Fists and Sanguinius of the Blood Angels fought a desperate rearguard action to halt the force pouring through the breach. Angron of the World Eaters slew loyalist Space Marines by the dozen, while Mortarion of the Death Guard led his pustulent Plague Marines into the thickest fighting.