Codex: Chaos Space Marines (2002), p5 — The Horus Heresy

The instant Isstvan III died the Emperor knew something terrible had occurred.

The Emperor dispatched seven Legions to confront Horus, to call him to account for his actions. The Warmaster's forces had redeployed to Isstvan V, where the first wave of loyalists made planetfall. The details of what has become known as the Isstvan V Drop Site Massacre are vague, for only a handful of Space Marines survived, and their descendants will not speak of it. According to the Mythos Angelica Mortis, the Raven Guard, Iron Hands and Salamanders Legions made up the first wave of the action, and were caught off guard by the ferocity of the traitor counter attack. As the first wave became pinned at the drop site, they attempted a breakout, only to discover that the four Legions of the second wave, listed in the Libra Historica as the Iron Warriors, Emperor's Children, World Eaters and Death Guard, had betrayed them.

The loyalists were slaughtered almost to a man, trapped between the armies of Horus and the newly revealed traitors of the second wave.