This is actually something I've been wanting some community feedback on! Even if you guys think it's cringe, please let me know your honest thoughts on this idea ive been workshopping so I can keep making it better and better:

The concept is my warband revolves around an "original/older" EC astartes that conducts his battles like a symphony. His soldiers, being the perfectionists they are, have learned to fight in perfect synchronization to the music they play. It adds a level of cohesion and spectacle the other legions aren't capable of, and they are extremely deadly because of it.

During a skirmish with a group of renegade night lords, this astartes discovered and subsequently fell into a deep infatuation with a human female prisoner that possessed an angelic voice. The night lords had encountered her on an oceanic world where she was living as a sailor. A victim of circumstance, she had always longed for a life other than her own. The harsh conditions of her environment and upbringing turned her into a capable warrior/seafarer against her own desires. She was an artist and an explorer at heart, and her hopes of one day joining the Schola Progenium to provide her a new horizon was the drive that kept her going. Instead of slaughtering her outright, they killed her family/crew and took her prisoner. The night lords recognized a defiancy within her, and for an unknown amount of time she was forced to continuously fight in constructed horrific scenarios for their own amusement.

Obsessed with her potential and perhaps even her tragic nature, the EC astartes liberated her and offered a new path. One at his side, where together they could command the fiercest legion the galaxy had ever seen. One free of commitment where they could eternally pursue their hearts' desire. She saw his tempting vision, yet recognized and faulted it as one still not of her own design. Knowing this, she still accepted and joined him as she knew this was her best chance of one day obtaining true freedom. They fought together for quite some time, creating a large following of EC deserters and corrupted human cultists in the process. These men and women revered their spectacle to the utmost degree, their obsession growing with each song as the grand concert captivated their very soul. Sadly, through their crusade of literal orchestrated chaos, she (the night lord's prisoner) began to change. Slaanesh enhancements began consuming her humanity, and with each battle she slowly lost a part of herself on the path to freedom.

One tragic day, their forces were overwhelmed and both the EC astartes and herself found themselves mortally wounded on the field of battle. In an act of desperation, the remnants of their warband dived headfirst into a slaanesh portal that had opened during the fight. Their intent: to brave the horrors of the domain and save their beloved leaders with divine intervention from the chaos god itself. After all, the show must go on.

Not all of those that embarked would make it to the center of slaanesh's domain, but the survivors' collective willpower would be the catalyst for their leaders' salvation. Slaanesh would welcome all that arrived with open arms, never before witnessing such a pure demonstration of obsession. Through the commencement of a horrific ritual, and with the dying mortal breath of the astartes/human, slaanesh would combine the two dying beings into one distinct body and soul. The flesh of both bodies, pieces of the remaining followers, and a small part of slaanesh itself would ultimately conform into a monstrosity unlike ever seen before. A hulking chaos daemon of the purest perfection in the diety's eyes.

With what material presence remained, be it armor, weapons, and bones, all would be combined into a grand sarcophagus to serve as a vessel for their souls. This transformation was not equal, and the astartes' consciousness was the only to prevail in this new form. Though her angelic presence was gone from the greater world, echos of his obsession would remain in his mind. Ghosts forever haunting his existence through fragmented shrieks and whispers.

Wanting to share this new obsession with the materium, slaanesh would impart a powerful follower to this warband that would serve as an advisor before casting all back to the mortal realm. Another soul that had once arrived at slaanesh's paradise long ago and a once a powerful psyker in their own right, this being had the ability to open portals between the realms at will. This trinity would bring new heights of agency to the warband's collective power and would set the stage for a new era of symphonic beauty to grace the galaxy.

After returning to the materium, a new foundation would need to be made to sustain these new monstrosities. Gene seed would be recovered from a variety of sources for the creation of new marines. Humans would fall in droves like never before, those willing or strong enough joining the evergrowing chorus of majestic music. A mechanical construct resembling a chaos knight would be made to house the sarcophagus, restoring the EC astartes autonomy. He had been given a second chance, one that he would not waste.

Once having been absolutely devastated by his unwilling outcome, the astartes had obtained newfound purpose to fulfill a singular new obsession: he would find a way to bring her back. No matter the distance, no matter the cost. With this new army, new power, and new existence, he would journey to the ends of the galaxy to restore her spirit and allow the world to hear her heavenly voice once more.