Introduction
First encountered by Imperial forces not long after the emergence of the race of xenos colloquially known as 'ponies' among the Protectorate forces, this highly unusual warband of Heretic Astartes follows the banner of the Black Commissar Ciaphas Cain, and is noteworthy for having a close association with the Protectorate's pony members. Since their first appearance, they have been involved in numerous skirmishes and a smaller number of pitched battles conducted across Cainite space and the wider Damocles Gulf.
This warband was previously known as the "Iron Warriors 38th Company" Rogue Chaos fleet, but changed its name to "Knights Harmonic" after it declared its newfound allegiance to the Cainite Protectorate. Since then, the Knights Harmonic have expanded to a force almost a thousand Astartes strong, not counting the numerous ponies and unaugmented humans that serve as support personnel and supplement their forces on the battlefield. They also appear to have close ties to the BORG's hereteks.
Warband Composition
The Knights Harmonic are composed of Marines drawn from eight out of the nine traitor legions (1), though former World Eaters, Iron Warriors, and Thousand Sons feature most prominently in their ranks (2). It is worth noting that such a combination of wildly disparate Heretic Astartes into a single Warband usually results in a sizable amount of internal conflict and dissension among the ranks, yet this Warband has managed to somehow defy this trend; in fact, for followers of the usually-fractious Ruinous Powers, the Knights Harmonic work extraordinarily well together, with infighting essentially non-existent.
The Knights Harmonic are also known for using Rubric Marines as frontline troops, like the Thousand Sons. Unlike the Thousand Sons, Rubric Marines present among the Knights Harmonic display an abnormal level of sentience and self-direction, operating independently of their Sorcerer commanders, and shouting orders and warnings to those around them. Imperial forces who've approached these Knights Harmonic-aligned constructs during times of (short-term, under duress) truce have even started and sustained extended conversations with them as if they were ordinary Astartes (3). If information gleaned from these heretics during such conversations is to be believed, these 'awakened' Rubric Marines are also effectively immortal, by virtue of being able to somehow 'transfer' to a pristine, unoccupied set of Rubric Marine armor when the one they are currently using is sundered.
Dissection of the few corpses of full-blown Heretic Astartes from this warband that we have managed to obtain reveals further abnormalities, this time in the biology of the non-Rubric Marines. The mutations members of this Warband undergo are unusually predictable and uniform in nature, so much so that one might even mistake them for organic augmentation comparable to the gene-seed implants all Space Marines receive if not for their obvious association with the corrupting forces of Chaos. The number and nature of these 'augmentations' that members of the Knights Harmonic develop is still undergoing investigation, but three of the most common have been identified so far; a streamlined, psychically active nervous system that increases their reflexes and agility to levels comparable to those demonstrated by the Craftworld Eldar, bones laced with crystalline alloys which appear to enhance their connection to the Warp while also insulating them against the usual perils associated with such a connection, and an organ that is in essence a more powerful version of a Belisarian Furnace gene-seed organ, able to remain active nearly constantly and allowing Knights Harmonic to rapidly heal from all but the most grievous of wounds in a matter of moments (4).
1. The absent legion is, of course, the Death Guard, with the Cainites' antipathy to the followers of Nurgle being well established. While the Iron Warriors 38th company were previously headed by Nurgle-aligned individuals, said individuals appear to have abandoned Nurgle as their chief diety. How they managed to get away with that is unknown at this time.
2. There is absolutely no evidence indicating the presence of renegades from loyalist chapters among the Knights Harmonic, and anyone insisting otherwise is a traitor spreading heretical lies who should be executed with all possible haste.
3. While such fraternizing is obviously heretical, we have allowed such infractions to slide if the Imperial doing the fraternizing is doing so for the express purpose of gaining much-needed intel on this highly unorthodox warband.
4. Belisarius Cawl has caught wind of this, and is having a massive hissy fit over the fact that we won't let him study the corpses of any members of the Knight Harmonic. I strongly recommend that we do not give into the Archmagos Dominus' irate demands, for reasons that should be obvious.
Combat Doctrine
The Knights Harmonic, in keeping with both their nature as a highly-diverse warband and the combat doctrines of the three dominant Legions among their ranks, pursue an aggressive and ruthless combined arms approach more akin to that utilized by the Ultramarines (5) or Imperial Fists than the tactics most other Chaos Warbands rely upon, with a particular emphasis on the excessive-yet-precise application of extreme-but-controlled violence to strategically advantageous points.
That odd, somewhat contradictory statement at the end of that last paragraph handily sums up what truly separates the Knights Harmonic from other Warbands; for better or worse, they show a surprising level of restraint on the battlefield, focusing all their energy on their strategic objectives and anything that stands between them and said objectives. When they appear on a planet, thousands of civilians and soldiers that would have been slaughtered wholesale by any other Chaos Warband are often allowed by the Knights Harmonic to walk away with their lives, sanity, and freedom intact, the reduced losses in manpower counterbalanced by considerably increased losses in infrastructure and war material. Their combat zones almost always lack traces of the Warp-based corruption and heretical rituals typical of most Chaos legions, life-giving infrastructure within those combat zones — water, food, hospitals and so forth — are often left completely untouched, and the sites that have fallen victim to their attacks and haven't been annexed by the Cainite Protectorate immediately afterward have been cleared as fit for rebuilding. This is in spite of the high number of powerful psykers among their ranks and heavy reliance on immaterium-borne sorcery, which are well-known for their corrupting effects. Why they are so dependent on such powers yet don't spread the otherworldly contaminations usually associated with them is unknown.
5. As mentioned earlier, there is no evidence that Astartes from loyalist chapters have defected to the Cainites, and the multiple sightings of loyalist sigils and colors among the Knights Harmonic can only be an Alpha Legion counter-intelligence tactic intended to sow discord amongst the loyal followers of Him on Earth.
Wargear
Despite lacking the support of a forge world or the true Adeptus Mechanicus, the Knights Harmonic are frighteningly well-equipped, suffering none of the material shortages and equipment degradation that constantly plague other traitor Legions on top of possessing a truly stunning assortment of pony-made xenotech at their disposal.
The Knights Harmonic all utilize a pattern of power armor unique to them, similar in design and appearance to 'Corvus' pattern MK-VI power amor aside from a few glaring differences. The first such difference is that this pattern eschews Ceramite plating for armor in favor of a crystalline, plasteel-like alloy that is far more durable than Ceramite while being significantly lighter. This pattern is also fitted with powerful thrusters that amplify an Astartes not-insignificant mobility, which combine with the armor's more compact, streamlined design to grant its wearers agility and speed on the battlefield comparable to that enjoyed by Tau battlesuits. Despite using sets of power armor entirely divorced from patterns typical of most Traitor Legions, members of the Knights Harmonic often choose to paint their armor in colors typical of the traitor legions they hail from (6), with their shoulder pauldrons painted with the insignias of their original legions.
The Knights Harmonic are also possessed of a distressingly diverse array of weaponry they can field, ranging from Volkite Weapons (7), highly advanced Plasma Weaponry, Close Combat weapons outfitted with masterwork Power Fields, and other, more exotic devices of Imperial origin, to various xenotech weaponry of Tau, Eldar, and Votann origin that are obviously indicative of collaboration with the perfidious Xeno species mentioned above (8), to exotic and incomprehensible artefacts devised by the BoRG hereteks and their pony associates. The truly stunning variety of weapons they can choose to field makes their capabilities on the battlefield all but impossible to predict until mere minutes before the shooting starts.
6 This cannot be emphasized enough; loyalist defectors to the Knights Harmonic DO. NOT. EXIST. No loyalist worthy of the title would ever do anything with those blasphemous perversions of the terran miniature horse's taint-free form other than shoot them dead with all possible haste, and anyone trying to claim otherwise is to be shot on sight for not only being a traitorous heretic and spreader of lies, but a complete and total brain-dead moron as well for failing to realize this obvious common-sense fact about Loyalist Space Marines.
7 These ancient, highly treasured Dark Age of Technology-era thermal ray weapons are somehow standard-issue weaponry among therank-and-file Tactical Squads of the Knights Harmonic.
8 This is not the case for their usage of Gauss Flayers and other Necron weaponry, as it is obviously impossible to form any sort of alliance with the necrons.
Conclusion
Do not let their relative youth in comparison to other Warbands fool you; while they may not be able to corrupt our purity and twist our motivations the way most Heretic Astartes are, they more than compensate with unbelievably high-quality wargear, a truly surprising amount of discipline and skill, and a level of tactical flexibility not even the Ultramarines can replicate. Whenever you encounter these foul heretics, exterminate them with all possible prejudice and haste. Ideally before they've served you your own internal organs on silver platters.
Thought for the day: Aliens never seek anything other than the complete and utter annihilation of mankind, and are never deserving of anything other than the slowest, most painful, most horrific form of death you can offer.
"...served you your own internal organs on silver platters? What do they think we are? Monsters?"
"It's a figure of speech, Twilight. Roughly equivalent to saying you got your flank kicked."
"Oh. Though that "Thought for the day" does kinda imply they see us as monsters-"
"Anyway, where'd you get this?"
"Trade secret, Ciaphas."