The Immortals have a famed and storied history, but have most recently gained renown for their bitter defense of the Bactria Sector against the Auric Tide. The chapter was nearly destroyed in the conflict, with barely a quarter remaining at its end, but has continued to guard the Bactria sector against foreign threats.
The Immortals Space Marine Chapter was founded in the 22nd Founding after the 5th Black Crusade (723.M36) ravaged the defenses of the Eye of Terror. Although the Imperial forces had managed to repulse the heretic forces of the Despoiler, many traitor warbands had used the Black Crusade as an opportunity to escape from the Eye of Terror and ravage the surrounding Imperial sectors. With each passing month, these warbands ventured further and further from the Eye of Terror, and, with the bulk of the Imperium's troops engaged on the Cadian front, few forces had been available to hunt them down. By the end of the Black Crusade, dozens of sectors had been overwhelmed.
The Immortals were created as a fleet-based chapter, tasked with pursuing and annihilating those traitor forces who had escaped the Eye, taking particular interest in the forces of the Black Legion. The Chapter has since dedicated itself to bringing retribution for the myriad sins of the traitor legions, and vengeance for those slain at Abbadon's hand. With each passing century, that list grows longer, and the Immortals' fury deepens.
The Immortals' first major engagement was in the 6th Black Crusade (901.M36), scarcely a hundred years after their founding. Caught in the midst of deploying to the Cadian Gate, the Immortals found themselves in the neglected Sorean Sector in the midst of a major Traitor Legion offensive. Repeating their tactics from the Fifth Black Crusade, the Black Fleet tied up the majority of Imperial assets in the Cadian Gate, while dozens of raider and reaver warbands escaped into Imperial space proper. The Immortals lead a bitter three-year void war against the 17th Legion warband, the Radiant Claws, which had come to the Sorean sector in the hopes of it being undefended. Lead by the Battleship "Virulent Grace," the Radiant Claws plunged into the sector, only to be savaged by a spirited counterattack lead by Chapter Master Tyndareus. This initial engagement saw the Immortals lose a third of their strength, although the Virulent Grace was also badly damaged. As the Radiant Claws continued their reaving of the sector, the Immortals continued to bitterly resist. Only when aid came from an Imperial Navy Battlegroup and the Red Talons Astartes Chapter were the Radiant Claws repulsed, but to the chapter's eternal shame, the Virulent Grace, and it's commander managed to escape back to the Eye of Terror.