Battle of Gate Forty-Two
The Akum-Sothos Cluster had been brought to compliance by the Luna Wolves in the opening years of the Great Crusade, but its people had since fallen to mass-psychosis and violently revolted against the authority of Terra. It was determined that a Xenos parasite which matured within the eye sockets of its host had been behind the riots. The parasites gained rudimentary control of their hosts as they matured and formed what amounted to an alien gestalt cabal of primary hosts dubbed the 'Unsighted Kings'. Newly ascended Warmaster Horus refused to see a cluster of worlds he himself had once conquered slip from the Imperium's grasp, so he vowed to retake the Cluster no matter the cost.
The Warmaster planned to launch a rapid blitzkrieg strike by the Luna Wolves, Space Wolves, Iron Warriors, and Raven Guard that would converge on the heavily fortified lair of the Unsighted Kings before a final overwhelming assault was launched across the entire Cluster.
After conquering the outer worlds of the Cluster in a matter of weeks, Horus called a council of his brother Primarchs. He called for the Raven Guard to make a frontal assault onto the guns of the defenders of Gate Forty-Two. Corax argued against this waste of resources and squandering of his warriors lives, countering with a strategy of his own. He proposed that his Legion should draw off enemy forces in a series of feints, allowing the three other Legions to overwhelm the defenders who remained at the Gate. However Perturabo suggested that Corax was simply seeking to avoid battle. The two nearly came to blows, and only the intervention of Leman Russ prevented bloodshed. However the Space Wolves Primarch told Corax to heed Horus' plan, as he had been the one the Emperor himself had put above any of his brothers. Reluctantly, Corax conceded to Horus' plan. He placed many of his Terran-dominated companies to the frontal assault, in particular those whose Captains appeared the most willing to support Horus' strategy.
The assault that followed was hailed as the Legion's darkest hour, a grim honour that saw entire Assault companies decimated in the face of heavy enemy defenses. Corax himself led the bloody attack that saw Gate Forty-Two finally taken. The Luna Wolves, Space Wolves, and Iron Warriors moved into the Unsighted Kings fortress soon after. Bitterly for Corax, the glory of slaying the Unsighted Kings themselves went to Horus and his Luna Wolves. But despite the victory thousands of Raven Guard, most of them Terran-born, had lost their lives.
The effects of the battle were far-reaching for the Raven Guard. Its numbers were sorely depleted after the battle, and the Legion numbered only 80,000 Marines after the battle. Corax removed his forces from Horus' command, bitterly swearing to never serve alongside the Warmaster again.
Another effect of the battle was that the Terran-born officers of the Raven Guard had largely been killed. Many of these officers had served under Horus at one point and some suspected their loyalties to their true Primarch was lacking. Thus the battle allowed Corax to consolidate his power over the Raven Guard, and erased the Luna Wolves influence over the Legion.