Massed deployment

This time, battle was met upon Agrellan, a world barren of human life but for twelve massive hive cities whose peaks scraped the cloudscapes of their own pollution. There, the Space Marines brought swift and terrible death to many a Tau cadre, with regiments of Astra Militarum and the towering Imperial Knights of House Terryn lending their strength to the Imperial cause. Once more, however, Humanity found the Tau an adaptable and cunning foe; Shadowsun's strategic brilliance saw all twelve Imperial hives conquered in a single day. Some were besieged, others were gutted due to rebellion or blown apart by weapons of planet-scarring force. The Space Marines found their supremacy on the battlefield likewise broken after the massed deployment of XV104 Riptides - giant battlesuits so powerful they tipped the balance of the entire war in the Tau's favor. Chapter Master Corvin Severax of the Raven Guard ordered the retreat, and Space Marines, Astra Militarum regiments and Imperial Knights alike made a fighting withdrawal.

Damocles: Kauyon pg. 6