He took part in the Heresy's Manachean War, where Deygen earned laurels for boarding actions at Port Maw. Afterwards, the Captain was tasked with delivering Warmaster Horus' Dark Compliance to Imperial worlds and to aid in this, he was given command of not only his 84th Company, but the Grand Cruiser Admonisher and an armada of thousands of Sons of Horus and Word Bearers, with millions of Traitor Solar Auxilia and Imperial Army troopers.
The Captain's targets were worlds along the warp routes between Cyclothrathe and Honorum, where he was to tithe materiel and flesh for the Cyclothrathe Dark Mechanicum. When it came to the backwater Forge World Ohmn-Mat, however, Deygen had a completely different purpose. Due to the scale of death enveloping it, as a result of Ohmn-Mat's civil war between Loyalists and Traitors, the world would be ideal for the Captain's Esoterists to deliver their dark benedictions to the Daemons of the Warp. For it to be completed though, more preparations needed to be done and Deygen allowed its civil war to rage on for two years, while his forces orchestrated surgically-precise butchery on Ohmn-Mat's surrounding Systems. All this was according to his Lodge Masters and Word Bearers Diabolists' schema, which had the 84th commit ritual killings on a mass scale, which rendered worlds into charnel tributes to the Traitors' glory. Deygen did so willing, however, as he was promised that doing so would grant him the entirety of Ohmn-Mat and earn an apotheosis, that would bestow arcane power upon the Captain and his Company.
At the close of 010.M31, the preparations were finally complete and Deygen led his fleet to the Forge World to conduct a Chaos ritual upon it. Ignoring the pleas of the paltry Traitor forces there, the Captain directed the 84th to the Loyalist Forge-Fane Thyaetira. It was the perfect altar for the Company's unfolding ritual, due to being marked by Ohmn-Mat's bloodiest battles. This continued as the 84th fought Thyaetira's Mechanicum forces to reach the Forge-Fane's vast Lacyraemarta harvesting precincts, where the ritual was to take place. They were harried all the way to the precincts by the Mechanicum, where the 84th were then attacked by an Iron Hands strike force, led by the War Leader Durante Osch. As the battle continued, the Company built sacrificial altars, where they began torturing any Loyalists they captured in order to summon forth Daemons on the Forge World. Osch's psychic senses allowed him to discover this and the Iron Hands would repeatedly attempt to rescue their captured brethren from the 84th's clutches. These attempts all failed, though one of the Legion's Contemptor Dreadnoughts managed to destroy an altar before it exploded. Thyaetira's Mechanicum forces also managed to destroy an altar, before the 84th's Chaos ritual was finally completed.
This allowed Daemon Brutes to appear, after they possessed the tortured Iron Hands and Mechanicum forces held in the sacrificial altars. The 84th's Esoterists also summoned forth Daemons and due to their ritual binding them to the Company, they only attacked the Loyalists. This soon overwhelmed Thyaetiran's defenders, even with War Leader Osch unleashing his psychic powers, for the first time since the Edict of Nikaea. With their defeat now certain, the Iron Hands leader bellowed a challenge to Deygen and the Captain accepted. The two would then duel each other, but with Osch already wounded from the battle and by using his psychic powers, Deygen easily defeated the Iron Hand. However instead of killing Osch, the 84th's Captain instead decided to gloat and show off to his Company. His mocking the Iron Hand for being twice-broken though, enraged Osch and gave the War Leader the strength to deliver a last blow that caved in Deygen's head. The Iron Hand leader then collapsed, as his killing strike had burned out the War Leader's augmetics. However despite his death, the Captain's body remained standing and just before the 84th moved into kill Osch, the Daemon Prince Samus possessed Deygen's remains. Though bound to the Company, like those summoned before him, Samus' arrival caused Daemons to materialize all over Ohmn-Mat, who fell upon both Traitors and Loyalists in an apocalyptic tide. Whether this was intentional or the result of Deygen's death is unclear, but any hope for survival for the Forge World's defenders was now extinguished.
Within Thyaetira the battle was nearly over, when a handful of its Mechanicum forces and Iron Hands were able to escape with War Leader Osch, who still lived. As they did so, the 84th and its Daemons began purging the Forge-Fane of any Loyalist that remained within it. Soon the Traitors found the last holdout within Thyaetira, where Osch's rescuers had brought the wounded War Leader. However as the location was about to be invaded and overwhelmed, the surviving Loyalists sought to deny the 84th their victory and activated the Iron Hands' Ceaseless Pyre Mimetic Annihilator. It was among the dangerous archaeotech that the Legion's Primarch, Ferrus Manus, had hidden within the Iron Hands' Vaults of Mimir and the Pyre's activation caused never-ending flames to envelop Ohmn-Mat. This scoured the Loyalists, Traitors and Daemons from the now former Forge World, but the large-scale destruction did not end there. Any voidcraft, planetary outpost or space-bound installation that had an active vox-link to Ohmn-Mat when the Pyre was activated, began to be burned by its flames as well, before then spreading to any of their open vox-connections. This only ended when its signal reached the edges of Ohmn-Mat's System, but by then the 84th's flagship, Admonisher, and Deygen's fleet had been destroyed.