Nathaniel Garro was the Captain of the 7th Great Company of the Death Guard. A veteran warrior who would remain loyal to the Emperor at the outset of the Horus Heresy, Garro is most notable for commanding the frigate Eisenstein through several perils in a successful attempt to escape the Isstvan system and bring warning of the Great Betrayal to Terra. Later he fled with the other remaing forces to the Hellcat Squadran Universe, to warn them of the uncoming doom.
Born on Terra, in the region of that world known as Albia, Garro enlisted in the Dusk Raiders Legion, as the Death Guard was known before its reunion with its Primarch, Mortarion, and its relocation to its new base on Barbarus. By the time of the Horus Heresy, Garro was one of the few originally Terran Marines still serving in the Legion, which caused a rift between him and his fellow Captains, who often called him "Straight-Arrow Garro." As Captain of the 7th Company, Garro was entitled to the honourific of "Battle-Captain", a tradition of the Legion dating back to the Unification Wars on Terra, when an officer of the XIV Legion distinguished himself in front of the Emperor, who bestowed the title on him as reward. Garro was proud to bear the title and approved of upholding the traditions of the legion, including the use of a housecarl, Kaleb Arin to act as his equerry. Despite the pride at he took in bearing these honours and occasionally having the favour of his Primarch Mortarion, Garro did not believe in looking for glory or further honour in battle, and did not go out of his way to impress. He believed above all in simply doing his duty to the Emperor.
A veteran of many campaigns, Garro had great experience of both enemy forces and his fellow Legions. Notably, he knew Captain Garviel Loken of the Sons of Horus legion from fighting alongside him during the Krypt campaign against Orks and was the sworn honour-brother of Captain Saul Tarvitz of the Emperor's Children. Both marines had small eagle emblems carved on the vambraces of their armour in such a way that, were they to clasp hands, the eagles would meet. Their closeness was formed during the Preaxior campaign, where they had saved each other's life. Garro distinguished himself when fighting alongside a detachment of Sisters of Silence at the end of the Great Crusade; their leader, Amendera Kendel promised to present his name to Malcador the Sigillite in recognition of his dutiful service. Similarly, she spoke well of him to Mortarion, who rewarded his Battle-Captain by selecting him for the tradition of sharing his cups. Mortarion also chose this moment to subtly sound Garro out about where he and his men might position themselves in the face of rebellion against the Emperor. Upon learning that Garro was unwilling to join the warrior-lodge within the Death Guard, Mortarion decided to take Garro to the meeting with Warmaster Horus at the initiation of the Isstvan campaign as his equerry, in an attempt to ensure Garro's loyalty. The 7th Great Company were part of the taskforce assigned to secure Isstvan Extremis, the outermost world of the Isstvan system, ahead of the Isstvan III massacre. During this battle, Garro was wounded by the enemy leader and lost most of his right leg, necessitating an augmetic replacement. The wound might have killed him, especially when his own company's Apothecary, Meric Voyen, was too far away, but Garro's life was saved by Apothecary Fabius of the Emperor's Children (a rather cosmic irony, given their later roles on opposing sides of the Heresy).
The wound meant that Garro could not be declared fit for combat duty, and he was instead posted to the Death Guard frigate Eisenstein alongside Commander Ignatius Grulgor. It came to pass that Garro was in command when Saul Tarvitz made his desperate attempt to reach the surface of Isstvan III and warn the legions there of Horus' betrayal. Garro aided his honour-brother in reaching the surface and so learned of the beginning of the Horus Heresy. With the betrayal on the planet came also betrayal in the stars, as Commander Grulgor and his detachment of Marines attempted to kill Garro and the men of the 7th attending him. Mainly thanks to the sacrifice of Kaleb Arin, Grulgor and his men were defeated. Not long after, the Eisenstein took aboard other victims of the betrayal, including Iacton Qruze from the Sons of Horus and Remembrancer Euphrati Keeler, whose conversations with Garro in the time afterwards strengthened his belief in the Emperor's divinity. Attempting to flee the system and warn the Emperor of Horus' betrayal, the Eisenstein was crippled by the Death Guard battleship Terminus Est and barely made the jump into the Warp. Once there, she attracted the predations of Chaos God Nurgle. The Chaos power resurrected Grulgor, his dead Marines and the ship's crew who had sided with him, creating some of the first Plague Marines. The ensuing battle between infected Warp creatures and Loyalist Marines resulted in the death of the ship's navigator and forced the Eisenstein to make an emergency exit from the Warp. Stranded hundreds of light years from inhabited space, Garro ordered the Eisensteins captain to overload her warp engines and jettison them into space. The ensuing explosion echoed across the warp and acted as a beacon for Rogal Dorn and the Imperial Fists' fleet, which had been becalmed by Warp Storms. Dorn rescued Garro and his men and took him back to Luna aboard the Phalanx. Even upon reaching Luna, and with the news of the betrayal delivered, Garro was still not safe, as one of his marines, Solun Decius, had become infected with Nurgle's Rot and fell to Chaos. Garro was forced to battle him throughout a Sister of Silence keep and out onto the surface of Luna itself. He eventually bested the daemonic ruin of his comrade and banished the entity he had become.
Garro was infuriated to be kept on Luna, a virtual prisoner, while the greater war was raging throughout the galaxy. Malcador offered Garro a new role as his agent, whose first mission was to gather a group of Astartes warriors from across the galaxy and bring them to Terra, for the sake of "the future." Garro accepted.The first journey of Garro's quest took him to Calth, in the midst of the Word Bearers' surprise attack on the Ultramarines. There, he recruited Tylos Rubio, a powerful Codicier. Another warrior was Macer Varren of the World Eaters, who had also abandoned his legion after refusing to turn against the Emperor. The last warriors Garro was ordered to recruit were Ahriman fo the Thousand Sons, his friend Tarvitz of th Emperor's Children and Garviel Loken of the Sons of Horus, who were still alive on the surface of Isstvan III. Although Loken had been driven nearly mad by the horrors of the massacre, Garro managed to bring him back to himself, and returned with his other brothers to Terra to learn Malcador's true purpose in gathering them together. It was evident that their forces were soon to be defeated, but there was a chance to rebuild, in another universe and so the group and their small fleet was sent away. But again the Terminus Est intercepted them and badly damaged their ships.