Julius Kaesoron was the First Captain of the Emperor's Children Legion during the Great Crusade and the Horus Heresy. He was present during many of the most infamous campaigns of the Heresy, including the Istvaan III Atrocity and the Drop Site Massacre of Istvaan V, and was one of the first Noise Marines, or "Kakophoni" as they were originally known, among the Heretic Astartes. Though it appeared that Kaesoron had met his ultimate fate on Istvaan V, it seems that Slaanesh, the Prince of Chaos, was reluctant to let such a promising servant die, and Kaesoron was later resurrected as a Daemon Prince.
He made his presence known in the material universe once again in 812.M41, on the Hive World of Shardenus during the Iron Hands' campaign to retake the rebellious Contqual Sub-sector. The Daemon Prince Kaesoron was confronted by a small group of Iron Hands Librarians and Lord Clan Commmander Arven Rauth himself, before finally being defeated and banished back into the Warp from whence he came.
Before his corruption by the Prince of Pleasure, Julius Kaesoron was a formidable Astartes of the IIIrd Legion and like his Primarch Fulgrim, was a great admirer of the arts. He had a particular fondness for reading poetry, especially the notable Remembrancer Ignace Karkasy's Imperial Cantos. He also had a love for music and would often attend musical recitals in the auditorium called La Fenice aboard the Emperor's Children's flagship Pride of the Emperor. Kaesoron accompanied his Primarch Fulgrim during the Cleansing of Laeran and was present during the attack on that xenos race's capital city as the genetically-modified serpentine Laer warriors died to protect their central temple which was dedicated to the worship of the Chaos God Slaanesh. Like many of those who entered the temple, Kaesoron was enraptured by the dazzling sights and strange discordant music as the cacophonous flood of sensations assaulted him with a surge of light and noise. After his return to the fleet in orbit, Kaesoron continuously sought ways to stimulate his senses in an effort to recapture what he had felt in the Laer temple, both on and off the battlefield. As their Primarch fell to the whispered temptations of the daemon-possessed sword he had taken from the Laer temple, the Emperor's Children Legion continued its descent into a downward spiral of Chaos corruption marked by the affected Astartes' insatiable desires to satisfy their hedonistic wants.
During the Laeran campaign, the IIIrd Legion's Chief Apothecary Fabius discovered upon examining some of the corpses of the Laer that the serpentine creatures had been engaged in extensive genetic engineering to perfect their species, creating a multitude of different castes who were genetically designed to best serve their intended function in Laer society. Bringing his findings to his Primarch, Fabius explained that the Laer were not so dissimilar to the Emperor's Children in their approaches to perfection. He put forward the hypothesis that what the Emperor had crafted when He had created the superhuman enhancements of the Astartes was incredible, but Fabius believed that it might be possible to go beyond what the Emperor had accomplished. Fulgrim inquired if Fabius truly believed that he could enhance the gene-seed of the Astartes. The Apothecary admitted that he did not know for certain, but he believed that they had to at least try, for by doing so they would move closer to perfection -- and only by imperfection would the Emperor's Children fail the Emperor. The Primarch agreed with the Apothecary's proposition and gave Fabius free leave to do what needed to be done. Through Fabius' ministrations, the Astartes of the Emperor's Children began undergoing surgical and chemical enhancement of their transhuman physiology. Kaesoron willingly underwent these criminal and outlawed medicae surgical procedures which would eventually spread cancerously through the IIIrd Legion as the power of the Warp reached out to swallow the Emperor's Children.
Just before the campaign began on Istvaan III, Kaesoron accompanied his Primarch to the Iron Hands fleet, in an attempt to persuade Ferrus Manus to join the Warmaster Horus' cause. But the proceedings did not go well, and the confrontation ended in violence. At a given signal by the Phoenician, the accompanying Phoenix Guard, Fulgrim's personal bodyguard, slew The Gorgon's honour guard, the Morlocks, outside the Primarch's chambers. Julius Kaesoron struck out against his counterpart amongst the Iron Hands, First Captain Gabriel Santor, and very nearly killed him with his pair of Lightning Claws. Taking their leave, the Battle Barge Pride of the Emperor and its Escorts opened fire upon the warships of the 52nd Expeditionary Fleet. This surprise attack crippled the Iron Hands Legion and provided a distraction while Fulgrim and the Emperor's Children fled into the Warp to rendezvous with the rest of their 28th Expedition in the Istvaan System.
During the lull between the Istvaan III Atrocity and the upcoming campaign on Istvaan V, Kaesoron along with the bulk of the III Legion's officer corps attended the fateful performance of the famed composer Bequa Kynska of Terra aboard the Pride of the Emperor in the La Fenice auditorium. The theatre had been garishly transformed into a pale likeness of the corrupted Slaaneshi temple on Laeran. Accompanying the Emperor's Children to the surface of Laeran, Kynska had been touched by the Chaos corruption of that foul place, becoming enamoured by the sights and sounds she experienced within the Laer temple, and she sought to compose an orchestral piece that she believed would recapture the wondrous sounds she had once heard in that place of Slaaneshi corruption. Her ultimate masterpiece was a symphony she named the Maraviglia and which she performed for Fulgrim and all the assembled Astartes of the Emperor's Children and their mortal support personnel within La Fenice. To recreate the sounds she had heard, Kynska created new musical instruments whose sonic powers could also be used for destruction when employed by an individual already corrupted by Slaanesh. As the Maraviglia began, the cachophony of sound unleashed by these instruments acted as a sorcerous ritual that opened a link between realspace and the Warp and allowed the power of Slaanesh to directly touch the audience. During the "performance" it was noted that the musical instruments were able to produce effects variously disorienting, stimulating and downright murderous.
Chaos mutations ran rampant through the audience and Astartes and mortal humans alike were so overwhelmed by sensation and uncontrollable emotions that they unleashed an orgy of both sensual hedonism and the most base form of murder upon one another. Ultimately, the music summoned five Lesser Daemons of Slaanesh known as Daemonettes from the Warp who possessed the bodies of Kynska and several of her singers and joined in the slaughter. During this part of the concert, several Emperor's Children Astartes, including Kaesoron, left their seats and took up the instruments to try and keep the Chaos music playing and in the course of their untrained fumblings with the instruments discovered that they could unleash waves of destructive sonic power filled with the strength of Chaos. These Astartes became the first Noise Marines, who would eventually take to the field on Istvaan V wielding this strange, new weaponry as a new unit of the III Legion called the Kakophoni under the command of Julius Kaesoron. It was during this performance in La Fenice that the Emperor's Children finally gave themselves wholly, both body and soul, to the Prince of Pleasure as his most dedicated servants.
During the Drop Site Massacre on Istvaan V, Kaesoron participated very little in the actual fighting and instead took his time defiling the corpses of the dead Loyalist Space Marines, taking bits of them as trophies to wear on his armour. Things eventually came to a head when Gabriel Santor of the Iron Hands was confronted by Kaesoron once again during that battle, but the Emperor's Children First Captain no longer bore the proud countenance of a regal Emperor's Children Astartes, for he had been recently transformed into one of the first Noise Marines. Screaming like a madman, the insane Chaos Space Marine lept nimbly around the battlefield. In the ensuing battle the two First Captains were mortally wounded and left in agony, though for Kaesoron, this agony was experienced as ecstatic pleasure, a pleasure for which the Emperor's Chidlren First Captain thanked his counterpart, telling Santor that it had been an "exquisite" experience before they both died. Though it appeared that Kaesoron had met his ultimate fate on Istvaan V, it seems that the Prince of Pleasure was reluctant to let such a promising servant die, and Kaesoron was later resurrected and ascended to the exalted rank of Daemon Prince.
Kaesoron made his presence known in the material universe once again in 812.M41, on the Hive World of Shardenus during the Iron Hands' campaign to retake the rebellious Purge of Contqual Sub-sector. The Daemon Prince Kaesoron was foiled through the Iron Hands' valiant efforts, which prevented him from opening a Warp rift that would have transformed the Imperial planet into a Daemon World. Kaesoron was confronted by a small group of Iron Hands Librarians from Clan Raukaan and their Lord Clan Commmander Arven Rauth, before finally having his physical form destroyed by these servants of the Emperor, who banished his black soul back into the Warp.
- Fulgrim (Novel) by Graham McNeill
- The Horus Heresy: Collected Visions, pg. 194 (Artwork)
- The Primarchs (Anthology) edited by Christian Dunn, "The Reflection Crack'd" by Graham McNeill
- Wrath of Iron (Novel) by Chris Wraight, Chs. 19, 21-22
- Angel Exterminatus (Novel) by Graham McNeill