Lycus was a Marshal in the Imperial Fists Legion and was part of the Retribution Fleet sent, by his Primarch Rogal Dorn, to confront Warmaster Horus when the Heresy began. However due to severe Warp Storms, the Fleet was left stranded in the Phall System and they were later attacked by the Iron Warriors Legion, in what became known as the Battle of Phall. When the Fleet's Captain, Alexis Polux, later gave the order to retreat back to Terra, Lycus's Strike Cruiser, the Light of Inwit, was one of the lucky ships to escape from the battle. In Phall's aftermath Lycus began traveling to Imperium worlds and ordered them, to both fortify their defenses and to mobilize their armed forces, so that they could be dispatched to aid in the war effort against Horus. Not every world he visited proved to be loyal though, as was the case with the world Cantaridine, which was revealed to have joined Horus' side after being visited by agents of the Warmaster. Despite set backs like this however, Lycus continued with his efforts and later visited Tallarn, ordering them to do the same as the other worlds he visited. Though Tallarn was a likely target for Horus' forces to attack, Lycus was met with refusal from the world's representatives of caliphers and trade dynasties, who said they did not have the resources or materials to do as he asked. Lycus could see that Tallarn's Governor-Militant Dellasarius agreed with him, but years of ruling and dealing with the representatives, had drained the war hero of his former vigour. Just as Lycus feared he would have to matters into his own hands and dispatch the Governor and representatives though, the Iron Warriors Legion suddenly appeared and attacked Tallarn with virus bombs. As sirens began to blare, Lycus did not know of the virus bombs, but he knew the sirens meant an orbital bombardment was imminent and grabbed Dellasarius before making his way to a nearby underground shelter. This act saved both their lives as the virus bombs destroyed Tallarn's atmosphere and killed a majority of the world's population. Once the virus bombs had finished destroying Tallarn, Lycus and the Governor traveled to the Rachab Fortress, where other military survivors were located, and began organizing attacks against the Iron Warriors, who had invaded the devastated world. However, with Tallarn's Astropaths killed by the virus bombs and Lycus not being able to contact the Light of Inwit, it appeared as if the survivors of Tallarn would not be able to contact the Imperium for aid.
This all changed though, when the Fortress received a message sent out by survivors from an underground shelter in Crescent City, asking to be evacuated. Though the Fortress' forces at first ignored the message, as it would be too dangerous or worthwhile to risk venturing into Tallarn's poisoned atmosphere to save a few survivors, they immediately made contact with the Crescent City survivors when they mentioned having an Astropath within their group. After Lycus heard this, he knew this would be their last chance to send out a plea for aid and asked Dellasarius to organize a strike force for him to command; though no plan was made to actually evacuate the Crescent City survivors, as it was likely the Iron Warriors had also heard their message. When the strike force was assembled they left the Fortress and made their way to the survivors shelter, but just as they suspected they clashed with the Iron Warriors, who were also making their way to the shelter. In the ensuing battle, Lycus's strike force was heavily outnumbered and in the end, only the Marshal was able to reach the shelter and was greeted by Kulok, one of the survivors they had made contact with. With the strike force destroyed and the Iron Warriors bearing down on the shelter, Lycus asked Kulok to quickly take him to see the Astropath Halakime and then ordered the Astropath to warn the Imperium of Tallarn's invasion and to ask for aid in fighting the Iron Warriors. As the Astropath began sending the message, Kulok suddenly realized that the Governor had never planned to evacuate the shelter's survivors and had only been interested in reaching the Astropath, in order to send for aid. In spite of now knowing there would be no escaping from the shelter, Kulok told Lycus he would join the Marshal in defending the shelter long enough for Halakime's message to be sent. They both returned to the shelter's entrance, just as the Iron Warriors forced their way inside and as Tallarn's poisoned air poured into the shelter, Kulok was able to get five shots off before the deadly atmosphere killed him. Despite now being alone, Lycus fought on until he was killed by the Iron Warriors and though the shelter fell to the traitors soon afterwards, Halakime was able to successfully send out his message to the Imperium.