Battle of Davin's Moon
The assault, launched by the Sons of Horus under Warmaster Horus against the moon of Davin, was ostensibly to bring the system back into compliance from the rebellious Governor Eugen Temba. However unknown to Horus, the events had been orchestrated by the Chaos-worshiping Word Bearers. Word Bearer's First Chaplain Erebus, a major adviser to Horus at this time, was key in the deception.
Upon landing on Davin's moon with 400 Astartes, 4,000 Imperial Army troops, and 3 Titans of the Legio Mortis, the Imperial forces found themselves on a corrupted world assaulted by horrifying Plague Zombie's and entities that worshiped a being known as Nurg-Leth. As the Astartes battled these creatures, Horus led an assault inside the remains of Tenmba's crashed starship Emperor's Glory. Inside, he encountered a Chaos-corrupted Temba armed with the weapon Anathame. After a fierce battle, Horus was wounded but managed to kill Temba, causing the Zombies to disperse.
The aftermath of the battle proved instrumental in galactic history, for Horus' wound soon grew worse and in desperation his Legion sought assistance from Davin's Chaos-worshiping Serpent Lodge. Horus was subsequently turned against the Emperor during his visions given to him by the Serpent Lodge, and the Word Bearers plan had worked perfectly.