Macer Varren - Mission to Isstvan: When Nathaniel Garro, the former battle-captain of the Death Guard Legion fled the slaughter at Isstvan III aboard the frigate Eisenstein, he brought word to the Emperor of the Warmaster's treachery and betrayal. Malcador the Sigillite, the acting Regent of Terra, met with the former Death Guard Astartes and offered him a chance at redemption.Under the Sigillite's own seal, Garro was tasked with finding 7 other Loyalist Astartes from amongst both the Loyalist and Traitor Legions who were utterly devoted to the Emperor and His Imperium in body and soul. Several of these Space Marines would eventually form the core of what would later evolve into the Grey Knights Chapter of Astartes.Garro discovered the former World Eater and offered Varren a new role as one of Malcador's operatives, but in truth, the duty sat poorly with him. Quick to anger like all World Eaters, Varren longed to be in the thick of the war, facing his former comrades. He lacked the cool detachment of Garro or the stoicism of his former Ultramarines comrade, Tylos Rubio.The last of the seven warriors that Garro was ordered to recruit was Garviel Loken, once a captain and member of the Mournival of the Sons of Horus Legion. Though it appeared at first that Loken had perished during the final bombardment by Horus' fleet of Isstvan III, he had somehow managed to survive on its blighted surface in the ruins of Isstvan III's capital, Choral City, for several Terran years after the Traitor Legions had moved on and the larger tragedy of the Horus Heresy had consumed the galaxy.The world of Isstvan III was now a desolate and barren graveyard planet inhabited only by Plague Zombies raised by the lingering taint of the Plague God Nurgle which had consumed the world after the Life-Eater virus had destroyed all life upon its surface.Loken had been driven mad by the horrors he had experienced and believed that he had been rejected by Death itself. He experienced a form of amnesia and could no longer recall his own name, taking upon himself the new identity of "Cerberus," and believing himself to be the sole remaining Loyalist and champion of the Emperor in the galaxy until he was confronted by Garro.After their arrival on Isstvan III by Thunderhawk, Garro, Rubio and Varren encountered a handful of what they believed to be Human survivors who spoke of a beast that hunted them. Taking matters into his own hands, Garro went to the lair of the so-called beast of Isstvan III, and on discovery of the long-dead corpse of Tarik Torgaddon, he was attacked by the unkempt, nearly-psychotic "Cerberus."During his fight with Loken, Varren urged the mad Astartes not to endanger the civilian refugees nearby. Garro wanted to put the insane Astartes out of his misery, but Varren argued that they had already lost too many brothers to Horus' treason.After a lengthy battle, during which Cerberus escaped, Garro and his companions discovered that the Human "survivors" of the virus-bombing of Isstvan III were in fact Daemonhosts who had been transformed into undead Plague Zombies of Nurgle when the virus-bombing had weakened the barrier between the Immaterium and Isstvan III, allowing the Plague Lord's influence to reign supreme.Garro and his Loyalist Astartes managed to defeat the foul creatures with the aid of Cerberus; however, once the undead had been destroyed, Garro and Cerberus had one more one-on-one combat match, as Loken perceived the group of Astartes as Traitors who had come to kill him.It was during this final fight that Garro managed to reach through the amnesia and fractured awareness of Cerberus to the core of the good man that remained deep within his psyche, reminding him of who he truly was, and that the Emperor still had a use for him in rooting out the corruption that was staining the Imperium.Aware of his true identity once more and how he had come to be stranded amidst the ruins of the Choral City, Garviel Loken joined Garro and his team. With his assemblage of warriors now complete, Garro and Loken left Isstvan III for Luna to receive their first mission from Malcador the Sigillite. Varren's fate after the Horus Heresy remains unknown in Imperial records.