During the opening days of the terrible conflict that would become known as the Horus Heresy in later years, the Warmaster Horus launched his treacherous attack on the world of Istvaan III against the Loyalist elements within four of the Space Marine Legions under his command, including the Sons of Horus, the Emperor's Children, the World Eaters and the Death Guard. The Ultramarines remained ignorant of this fact. While Horus made his plans for what would become the infamous Drop Site Massacre on Istvaan V, the Warmaster sent word to the XVIIth Legion's Primarch Lorgar that the time had come for his Astartes, the Word Bearers, to strike against the Imperium. The Warmaster was keenly aware of the bitter hatred that Lorgar had for his Primarch brother Roboute Guilliman and his XIIIth Legion,