Macer Varren

Mesa Varren was a former Captain of the traitorous World Eaters Space Marine Legion who managed to remain steadfastly loyal to the Emperor of Mankind when his Legion became corrupted and threw in their lot with Warmaster Horus during the Horus Heresy.

History
Mesa Varren served as a Captain of the feared and deadly XII Legion World Eaters. Always leading the bloodiest assaults and most vicious battles of the Great Crusade, Varren helped to contribute the World Eaters' reputation for bloodshed and fear on a mass scale. Varren was a warrior who bore the aspect of a veteran. He bore the scars of countless conflicts upon a chiseled, granite hard face. The stiff lines of Mesa Varren’s features mirrored those of other season veterans. Varren wore a tight cowl of dark hair and a ragged beard.

Like his fellow World Eaters, Varren possessed a short-tempered and was always eager to prove himself in battle, he possessed a strong code of martial honour. But unlike his traitorous brethren who spat on their oaths of fealty, Varren remained resolute and was steadfastly loyal to the Emperor. Varren had forsaken his own kinsmen after they swore fealty to the Warmaster Horus. The lord of the XII Legion, the gladiator Primarch Angron, sent his personal guard to kill Varren for his refusal to join the World Eaters in their sedition. Varren barely escaped this assassination attempt with his life. His heart embittered by the betrayal of this former brothers.

When Nathaniel Garro, a former Battle-Captain of the Death Guard Legion fled the slaughter at Istvaan 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 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. These Space Marines would eventually form the core of what would later evolve into the Grey Knights Chapter of Astartes, the Chamber Militant of the Inquisition's Ordo Malleus. 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 the World Eaters he longed to be out in the thick of the war, facing his former battle-brothers. He lacked the cool detachment of the Death Guard or the stoicism of his 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. Though it appeared that he had perished during the final bombardment on Istvaan III, he had somehow managed to survive on its blighted surface in the ruins of the Choral City for several years after the Traitor Legions had moved on and the larger tragedy of the Horus Heresy had consumed the galaxy. The Warmaster’s insurrection had long since moved onwards towards other sectors. The world of Istvaan 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', believing himself to be the sole remaining Loyalist in the galaxy and champion of the Emperor until he was confronted by Garro.

After their arrival on Istvaan 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 Istvaan 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's treason. After a lengthy battle, during which Cerberus escaped, Garro and his companions discovered that the human "survivors" of the virus-bombing of Istvaan 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 Istvaan 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 of Istvaan III, Garviel Loken joined Garro and his team. With his assemblage of warriors now complete, Garro and Loken left Istvaan III for Luna to receive their first mission from Malcador the Sigillite. Varren's fate after the Horus Heresy remains unknown.

Canon Conflict
A character by the name of Varren (no first name) was mentioned in the Realms of Chaos: Slaves to Darkness background books. In this version of events it was Captain Varren who led a delegation of loyal World Eater Legionnaires who fled Istvaan III to warn the Emperor of Warmaster Horus’ treachery after they seized control of the frigate Eisenstein. In the Horus Heresy Novel The Flight of the Eisenstein by James Swallow, Varren does not appear in the story. Instead it depicts Nathaniel Garro and his loyal Death Guard Astartes seizing the Eisenstein and fleeing to Terra.