Codex: Chaos Space Marines (2017), p9 — Heresy Unbound

Over one hundred companies drawn from the Emperor's Children, Death Guard, World Eaters and Sons of Horus Legions had already taken up arms against the rebellion on Isstvan III's surface at the time of the virus bombing. Horus had ensured their numbers were almost entirely made up of those Space Marines from each Legion who were still loyal to the Emperor, intending to wipe out those who had proved incorruptible in a single deadly stroke. Of these brave warriors, fully two-thirds miraculously survived the bombardment, thanks to warning messages received from the dwindling ranks of loyal comrades left aboard the orbiting fleet of the Warmaster. Angron, headstrong as ever, made planetfall at the head of fifty companies of Traitor Marines to slay these survivors. The ground war began in earnest, signalling the start of the greatest turmoil to ever have engulfed the Imperium.

As the forces of Horus launched their orbital barrage upon the loyal Space Marines on Isstvan III, Captain Garro, of the Death Guard, seized the frigate Eisenstein from the traitor fleet. His instinct had led him to warn the loyal warriors on the planet's surface of their imminent betrayal, and he was determined to flee the Isstvan system. Garro was unable to prevent the atrocities that followed, and his vessel was no match against the battleships of Horus' blockading fleet; all he could hope for was that the Eisenstein could escape from the Isstvan system and somehow find a way to reach Terra to warn the Emperor of Horus' treachery.