Garviel Loken was the Captain of the Luna Wolves (later Sons of Horus) Space Marine Legion's 10th Company during the last days of the Great Crusade and up until the Battle of Istvaan III that initiated the Horus Heresy. Loken became one of the leaders of the Loyalist factions of the Luna Wolves, Emperor's Children, World Eaters and Death Guard Legions that refused to betray the Emperor of Mankind and serve the Ruinous Powers of Chaos. Later, he was recruited from the undead-haunted ruins of the Choral City on Istvaan III by his fellow Loyalist Space Marine Captain Nathaniel Garro, formerly of the Death Guard Legion, to serve as one of the 8 Astartes who founded the elite Grey Knights Space Marine Chapter of what later became the Inquisition.
Loken was an unusually thoughtful, calm and rational Space Marine, referred to as "the quiet one" of the Mournival by Horus. While not a man of many words, he often surprised those around him with the depth of his mind and character, unexpected in one who had been bred for war. Like most of the members of his Legion, Loken was a native of Cthonia, the ancient Mining World near Terra where Horus had been found by the Emperor of Mankind in the early days of the Great Crusade. Unlike his fellow Astartes, Loken spent a great deal of his time reading and studying ancient writings loaned to him from the vast Librarium of ancient human texts aboard the Luna Wolves' battle barge Vengeful Spirit by his friend, the Imperial Iterator Kyril Sindermann. It is the insights gained from this study that enabled Loken to see the corruption of Chaos for what it was as it took hold of Horus and most of the Legion around him. It was his devotion to the truth at all costs, and his loyalty to the Emperor that ultimately placed him at odds with those who followed Horus into darkness.
Loken had many friends within the XVI Legion as well as outside it in the days before the start of the Heresy, an unusual quality for an Astartes, as the Space Marines of the Luna Wolves rarely took notice of the normal men and women many of their kind routinely dismissed as "mortals." He befriended the Remembrancers Mersadie Oliton and Ignace Karkasy. Oliton, a writer, followed Loken and often met with him in his personal armoury to record the takes of his experiences in battle. Karkasy was a poet who, after landing in trouble with the Imperial Army and nearly being beaten to death for openly speaking his mind about the potential immorality of the Great Crusade's conquest of human worlds, caught Loken's attention. Loken respected him for his unusual willingness to speak the truth even when he did not agree with his conclusions and the Luna Wolf captain sponsored him, allowing him to remain with the 63rd Expeditionary Fleet. Loken protected Karkasy, even when Karkasy's poetry became openly critical of the Astartes.
Another important figure in Loken’s life was the Imperial Iterator Kyril Sindermann, a member of the corps of Imperial propagandists who accompanied each Great Crusade Expeditionary Fleet to carry out the cultural incorporation of conquered human worlds into the Imperium of Man by spreading the Imperial Truth (which, ironically, was actually a lie concocted by the Emperor to weaken the powers of Chaos). Loken had often pursued in-depth discussions on a variety of intellectual topics, including religion, which had been dismissed as mere superstition by the rationalism of the official creed of the Imperial Truth. This association, along with his protection of the Remembrancer Ignace Karkasy, later led to strife for Loken with Horus and his Warmaster’s new confidante, First Chaplain Erebus of the Word Bearers Legion, as the Warmaster, often manipulated by the long-corrupt Erebus, turned against all the Remembrancers accompanying the 63rd Expeditonary Fleet, believing them to be of little use in his war against the Emperor, particularly as they began to tell the truth about the corruption induced by Chaos.
Captain Loken’s courage and strength made him an officer on the rise in the Luna Wolves. Loken’s close friend Captain Tarik Torgaddon mentored Loken in several aspects of his martial life and eventually was responsible for his induction into the Mournival, the group of informal advisers from the Luna Wolves Legion who served Horus as his closest companions. Loken was inducted into the Mournival after the Luna Wolves' campaign on the world of Sixty-Three-Nineteen, bringing it into Imperial Compliance. Loken also had the distinction of being the only member of the Mournival not to share a direct facial similarity to the Warmaster Horus. As a member of the Mournival, Loken was to be the Warmaster’s "naysmith," offering a counter-argument to those plans of Horus he found fault with and was intended to provide an advisor for Horus who possessed a calm and imperturbable nature. During this time Loken enjoyed a privileged position within the Warmaster’s inner circle.
When the Luna Wolves were conducting Imperial Compliance operations on the world of Sixty-Three-Nineteen at the end of the Great Crusade, a strange voice appeared in the vox radio communication devices used by the Luna Wolves, which was actually the voice of a Chaotic daemonic entity. The walls between the physical world and the Immaterium were unusually thin in that region of Sixty-Three-Nineteen where the remaining hold-outs to the Imperial conquest were holed up, though the Imperial forces during the Great Crusade had been kept deliberately unaware of the existence of Chaos or the terrible daemonic dangers of the Immaterium by the Emperor and his Primarchs. The voice only identified itself as "Samus" and warned against the oncoming invasion by the Luna Wolves. The Luna Wolves chased the source of the communication into a mountainous region of the planet. While there, Sergeant Xavyer Jubal of Loken's 10th Company became possessed by a Chaos daemon, and transformed into what was described by witnesses to the event as a monster attacking his fellow Astartes. After a fierce confrontation, Captain Loken was able to subdue and ultimately kill the deranged, mutated form of Xavyer Jubal. This confrontation would begin to stir within Loken a deep disquiet that perhaps the Imperial Truth was in error, as was everything he had been taught about religion and the supernatural being nothing more than superstitions rightly cast off by humanity.
After the terrible events on Sixty-Three-Nineteen, the Luna Wolves were sent to reinforce the Emperor's Children on a planet later nicknamed "Murder" by the Imperium for the sheer ferocity of the arachnoid xenos who called it home. The Emperor's Children had arrived at Murder in order to investigate the disappearance of a company of Astartes from the Blood Angels Legion. It was on Murder that Loken and his Mournival brother Tarik Torgaddon met and befriended Captain Saul Tarvitz of the Emperor's Children.
The relationship between Loken and his Mournival brother First Captain Ezekyle Abaddon was always strained as his calm, thoughtful personality constantly clashed with Abaddon's volcanic temper and gruff nature. Despite this, they stood together with Horus as he led an attack on rebel Imperial forces on the moon of Davin. When the Warmaster was mortally wounded in battle there by the Nurgleite former Planetary Governor of Davin, Eugen Temba, who used a Chaos-infused relic blade known as the Kinebrach Anathame to poison Horus with a Nurgle-spawned toxin that not even his superhuman immune system could defeat, it was Loken who vocally and absolutely opposed the idea of taking Horus to a Chaos temple known as the Temple of the Serpent Lodge on the world of Davin to be healed by shamans who were essentially Chaos Sorcerers. While Abaddon and Horus Aximand opposed him in their desperation to save Horus Lupercal's life by any means necessary, Tarik Torgaddon stood with Loken in believing that such a violation of the Imperial Truthcouldonly lead to even greater harm than the Primarch's death. The rift in the Mournival over this issue nearly came to violence, but Loken ultimately gave into the broader Legion's feelings on the issue and he chose to wait and see the result.
This attitude cost him the trust of not only his two Mournival brethren but also of Horus himself, who emerged from his healing at the hands of the Chaos Cultists of the Serpent Lodge a changed man, one riven by jealousy, egotism, hatred for his father the Emperor and the lust for absolute power, a man who had given his soul to the Chaos Gods in return for the powers of the Warp. It was at this time, after Horus' brush with death, that the Luna Wolves were renamed the Sons of Horus in order to clearly stake the Warmaster's claim to superior authority over all the other Primarchs. Now falling firmly into the grip of Chaos, Horus chose the world of Istvaan III to dispose of those members of the four openly declared Traitor Legions - the Sons of Horus, the Emperor's Children, the World Eaters and the Death Guard - that he could not trust to follow him or serve the Chaos Gods in his new quest to dethrone the Emperor and rule the galaxy himself. Loken and Torgaddon as well as approximately one-third of the 4 Legions' Astartes who were judged to be Loyalists were sent to the planet of Istvaan III under the pretext of putting down a rebellion against the Imperium of Man by a Slaaneshi-corrupted Planetary Governor only to be slaughtered by Life eater virus-bombs launched from the Warmaster's orbiting fleet. A last minute warning about the imminent betrayal from their friend Captain Saul Tarvitz of the Emperor's Children provided some of the Loyalists, including Loken and Torgaddon, with enough time to seek shelter and thus survive the resulting virus plague and the firestorm that followed that killed all of Istvaan III's 12 billion people. Even with Tarvitz's warning, at least 2/3's of the Astartes Loyalists did not make it to shelter before the bombs dropped, or emerged from the shelter too early, believing the attack to be over, only to be obliterated in the subsequent firestorm that ignited all the noxious, decaying gases released by the virus victims' bodies.
With Horus' betrayal of them and the Emperor now oblivious, Loken and Torgaddon and the other surviving Loyalists of their Legion took up the name Luna Wolves once again to symbolise their split with the Sons of Horus as they openly fought against the Warmaster's Traitor Legions. They fought using a defensive strategy intended to bleed the Traitor Legions for several months to allow news of Horus' betrayal to reach the Emperor. They managed to hold out on Istvaan III for almost 2 months before the final battle, in which Horus Aximand and Abaddon traveled to the surface to confront Loken and Tarik face-to-face. Loken fought bravely against Abaddon but he was critically wounded, and lay helpless in the final orbital bombardment of the planet by the Warmaster Horus' fleet, which brought the resistance of the Loyalists to its end.
However by some twist of fate, or through the will of the Emperor, Loken did not die in the bombardment. Months later, well after Horus' fleet had moved on to face the Emperor's Loyalists at the Drop Site Massacre on Istvaan V, where 4 more Astartes Legionsdeclared their open allegiance to Chaos and to Horus, Loken finally managed to dig himself out of the rubble of the Choral City, Istvaan III's capital city. Unfortunately, Loken's mind had been broken by the trauma of the betrayal and his wounds and he had lost both his memory and his identity. Instead, he took up a new role and named himself Cerberus after the ancient Terran legend of the hound who guarded the gates of the Underworld, because he believed that death had rejected him, the "monster" who now haunted the burnt and broken ruins of Istvaan III for over a year, killing the other survivors of the betrayal, believing himself to be the last Loyalist Space Marine in existence.
This changed when Battle Captain Nathaniel Garro, formerly of the Death Guard Legion and now a Loyalist Space Marine in the special service of the Emperor after his escape from Istvaan III on the Imperial frigate Eisenstein, returned to Istvaan III with the express purpose of searching for Loken as a recruit to join the organisation that Malcador the Sigillite intended to found on the Emperor's orders that would later become the Imperial Inquisition. They encountered a handful of 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". 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 once more faced one another in mortal combat.
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, 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.