Horus was considered by many of his brothers to be a failure. The poorest general, the weakest warrior, and the slowest conqueror. Few would have imagined an insurrection led by such a man would cause such untold damage to the Galaxy.
What Lorgar knew, in addition to his forbidden knowledge of The Warps, were two key truths. The first was that he knew his own weakness. The second was he knew the weaknesses of his brothers. Lorgar saw his brothers for how they truly were: arrogant, belligerent, mercurial, and foolish. He saw the resentment welling up from their "loyal" sons, and he saw the misery of the Imperium's people under the bootheel of The Emperor.
Lorgar could never have faced off against the Emperor or The Warmaster. But he also knew he didn't have to. Thus the Great Treachery, was not a grand battle of order versus chaos. The conflict was slow, long, arduous, and complicated. Those loyal to The Emperor were deprived of all the resources they took for granted: their unhindered access to the warp, the unquestioning compliance of their sons and subjects, and their access to reliable information and materials. Lorgar's Revolution to create Urizenesh left the Imperium stranded and confused, with many trapped within their corner of the unravelling empire, and others unable to tell friend from foe.
During the Desolation of Lorgar, certain forces, despite what alignment they claimed to have been, acted against loyalists and traitors alike. Often serving no one but their - or their god's- own interests.
Loyalist Factions
The Emperor of Mankind
These are the forces the Emperor had direct control over, and could (reasonably) rely on for the conflict
The Imperial Fists
The Praetorian legion, Rogal Dorn was always a reliable asset for the emperor and the warmaster. That was until his rivalry with Pertuabo came to a head, and he spent the better part of a decade hunting and besieging Perturabo's fortresses in what would become known as "The Battle of the Iron Cages."
The Blood Angels
As the other legions fell into chaos, Sanguinius and his Blood Angels took up the Burden of Command, leading the majority of the attacks in Horus' absence.
The Sons of Horus
While Horus was captured by his wayward Mournival, his loyal remaining sons continued the fight in his name. Led by Abbaddon, the only member of the Mournival to not become corrupted.
The Salamanders
The Salamanders were not regularly involved with most battles during the treachery. This is not for any lack of loyalty, but because of the state of the galaxy. Trapped in the Ultima Segmentum, Vulkan was essentially forced to lead his own war trapped between Luther's Imperium undivided, and Lorgar's chaotic forces. The Salamander space, a de facto petty imperium, regularly was aided by the stranded forge worlds and the Ultramari resistance in the fight against Luther and opportunistic Drukhari.
The Custodes
Originally created for the sole and single purpose of protecting the emperor, the disastrous early stages of the war saw the need for an additional mission for the Custodes. Lorgar's command of warp storms and ability to disrupt communications had led to the complete destruction of no less than 3 legions, and thrown the entire Imperial war machine in disarray. Convinced of the dire Circumstances regarding the treachery, Magnus, Malcador, and The Emperor were convinced that the webway gate was vital for victory in the war. And as such the Custodes in a mad dash to acquire the remaining materials to finish the webway project. The custodians spent the heresy travelling the craftworlds and blackmarkets of commorragh, obtaining artifacts and "persuading" eldar to aid in its completion.
The Dark Angels
The loyalist Dark Angels were absent for the early stages of the war, instead left stranded on a Demonic infested Caliban. Following their Rescue by the Vostroyans, they joined Sanguinius and Lupercal in the liberation of Segmentum Obscurus.
The Iron Hands
The Iron Hands became Sanguinius' most reliable asset in the heresy. Aiding the Warmaster Pro Tempore in most major conflicts.
The White Scars
Present during the Lorgar and Erebus' treachery in Segmentum Obscurus, Jaghatai Khan and his sons were able to use the power of the Dark Glass to navigate past the warp storms, rescuing Magnus and Leman Russ from the fall of prospero. Jaghatai was ordered by the emperor to recuse himself from the war, instead to use his artifact and his legion to serve as a messenger, as well as an escort for the custodes in their quest for the necessary pieces of the Imperium.
The Dusk Raiders
When news of Typhus's treachery was made known in the death guard, the ensuing events saw the Legion split into three factions. The Terran based Death Guard, under the command of Nathaniel Garro, mutinied against Mortarion's madness. Taking their old name of The Dusk Raiders, they returned to Terra to serve in Sanguinius's war efforts.
The Alpha Legion (officially)
Alpharius, became relied upon, first by Horus, then by Sanguinius and Malcador for communication as the communication networks of the Imperium unraveled. It was also Alpharius who helped coordinate the Custodes rush for materials to construct the webway.
The Legionless Primarchs
The power of the warp under Lorgar's command, and the scheming of Alpharius, caught the Legiones Astartes completely off guard. And while many suffered tremendous casualties. The Thousand Sons, Emperor's Children and Space Wolves were all but extinguished. Their surviving primarchs Magnus The Red, Fulgrim, and Leman Russ continued to offer their support to the war effort in other capacities.
Magnus The Red
In foreshadowing his later role, Magnus assisted the other chapters by lending his knowledge of the warp to aid against demonic forces. He also worked to aid his father in the completion of his great webway project.
Fulgrim
Having lost his home planet, and his legion, Fulgrim joined his brother, Ferrus Manus. Ferrus giving several companies to Fulgrim's direct command. Under Fulgrim's influence, the Iron Hands became more nuanced and more balanced of a legion, taking on some of the best traits of The emperor's children. Fulgrim would also act as a fortification and engineering expert in the absence of Rogal Dorn.
Leman Russ
Leman refused to be seen along with his brothers following what he saw as a personal failure on prospero. Donning Black unordained power armor, Leman Russ vowed to hunt down and exterminate every single one of his treacherous sons. Russ, reluctantly agreed to put off his hunt at the pleas of his brothers Dorn, Vulkan, and Magnus, not wanting to have another brother vanish. Leman served the war as a commander for the Auxiliaries, helping human forces survive the onslaught of horrors. In particular, Leman took a personal charge on aiding and organizing the abhumans of the Astra Militarum, ensuring their proper treatment and training.
The Ultramar Resistance
Lorgar's great opening salvo was the complete and utter destruction of Ultramar, Gulliman was killed, the realm under Chaos and Demonic control, and the few Ultramarines were scattered. However, as the war progressed those remaining were slowly able to regroup and act to reclaim their homeland.
The Ultramarines
After the Betrayal at Calth, the Fall of Maccragge, and The scouring of Ultramar, the once mighty legion was reduced to a few company captains who accompanied Tarasha's Migrant fleet. They refused to allow that to stop their service to the Emperor and fallen Primarch. Convinced of their Fathers new found divinity, the Ultramarines slowly recovered their strength as they liberated possessed ultramarines alongside the newly found Cult of The Aurelian.
The Ancillae of Avilla Ordinata
Commissioned by Tarasha Euten herself, dispossessed noble Avilla Ordinata of San Leor liberated her planet from the clutches of Lorgar's heretical imperial cult, and established the "Handmaidens of Tarasha." Her sisters joined with the Exiled Ultramarines and began a campaign to uproot Lorgar's heretical creed with the true faith of the Divine Gulliman.
Macer Varren's World Eaters
Originally part of Angron's Siege of Macragge, Macer and The Codeces of the Word Bearers were abandoned on the planet and left to die by their Genefather. His warriors defected instead aiding in the defense and final evacuation of Maccragge. Macer Varren became the first Living Saint of The Aurellian Cult, becoming known as The "Red Angel."
Kaspian Hect's Word Bearers
A loyalist word bearer, he along with Macer Varren and Fel Zharhost were among the last to see Roboute Gulliman alive, eventually joining the migrant fleet. Hect and His "Sin Bearers'' fought alongside the Ultramarines in liberating the Ultima Segmentum. After the Treachery, The followers of Kaspian Hect joined Ultramar as "The Will of Guilliman" chapter, and eventually settled on Prandium.
Fel Zharhost's Night Lords
Part of The Night Lord's detachment to the siege of Maccragge, he was abandoned when the Word Bearers and World Eaters turned on Curze's night lords. He joined Kaspian Hect and Macer Varren, rechristening his chapter the "Night Hunters." Following the Heresy, Zharhost joined the Ultramarines and became the Holy Seat of Nova Tholium.
Cawl's Mechanicus In Exile
Protesting the "neutrality" of mars, relatively unknown Archmagos Belisarius Cawl entered the fray of Mechanicus Politics to secede from Mars, declaring their disobedience to the Omnissiah heretical. Several of the forge worlds joined Cawl and fought against the schismatic martians and traitor forces alike.
Traitor Forces
Forces of Chaos Undivided (aka Lorgarites)
Those that eventually become the Black Legion, those who adhere to the Heresiarch Lorgar's vision of a United Force of the Ruinous Powers.
Lorgar
Heresiarch, Horus never joins the Great Treachery. Under Lorgar's leadership, the war is less of a grand battle, and more of a series of Demon-augmented Guerilla incursions. Which in some ways is more successful, but in others was a greater failure. In particular, Lorgar lacks the ability to unite the traitor forces, but instead manages competing factions.
Sons of Horus
Betraying their father at Isstivan, the Mournival and Erebus overtake his legion, becoming collectively known as "Horus."
Horus Meloncholis
Once known as Horus Aximand. Aximand takes the Anatheme blade instead of his father. Corruption heightens insecurity about being an underling. Convinces most of the Mournival and Sons of Horus to revolt and mutinies. It is ultimately him who begins the corruption of the mournival. His desires to be greater and grander than his father lead to him representing the will of Slaanesh.
Horus Phlegmatic
Once known as Garviel Lokken, he was the last to be corrupted. He is deceived by Erebus into attempting suicide with a possessed blade. Which corrupts him into a servant of Tzeentch.
Horus Choleric
Once called Ezekyle Abbadon, Horus Choleric was eager to join in the corruption. Almost too eager. And his bloodlust leads him down the path of Khorne.
Horus Sanguine
Once called Torik Torggedon, Horus Sanguine's Hopefulness turned to despair upon seeing the state of his genefather. He abandoned his loyalties and under the guidance of Erebus embraced the love of The Rotfather.
Perturabo
Following his being snubbed at Nikea, Perturabo serves "The Great Patron [Slaanesh]" along with his Iron Warriors, Now the Iron Craftsmen.
Angron/Lortara
Nurgle cures Angron's Butcher's nails and resurrects his gladiator friends. Angron serves as Champion of Nurgle, with Lotara ascended to Demon Prince-hood (known as Saporin)
Saporin and The Conqueror
Lotara Sarrin joins Angron in his worship of The Rotfather Nurgle. Ascending to the Daemonhood as Saporin, she represents Nurgle's will as a bringer of entropy. Her ship The Conqueror was remade along with her crew.
Alpharius (Officially)
Alpharius learns of the Great Schemer, and chooses to become a pupil of such a master overlord.
Imperium Undivided (Lutherites)
A concurrent and allied movement to Lorgar, the Forces of what would eventually become Imperium Undivided pursue a different strategy for securing humanity's place in both the Materium and Immaterium.
Luther
The Lion's Adoptive Father, Disagreements with the Lion expanded into disillusionment with The Imperium, and The Emperor's love of Genetically modified monstrosities over norms and codes. Rebelled against the Lion, along with several other Commanders who rebelled against their primarchs. Seek path between the Fanaticism of both Lorgar and The Emperor.
Ahriman
Outraged upon learning of Magnus' deal with Tzeentch, Ahriman joined with Luther, and left Magnus to die in Prospero along with Leman Russ. He is one of the major ideological founders of Luther's Imperium Undivided, and brings with him Magnus' Stolen Rubric and an army of Rubric Marines.
Typhus
When Mortarion Learned of Typhus' machinations, his rage reached the point of causing an inter legion civil war. The smallest faction was Typhus, but it was the one most personally biting to Mortarion. Typhus, and many of the Children of Barbaros abandoned Mortarion to continue to Serve Nurgle.
Fabius Bile
Founding members of Imperium undivided along with Luther and Ahriman. Unlike the others, did not bring any forces with him, but instead abandoned Fulgrim following the destruction of Chemos.
Nagoya Khan
The forgotten founder of Imperium Undivided, he led the warrior lodges against the khan, and prevented Jaghatai from involving himself in Prospero. In Luther's Imperium, he is just one of many Lodge leaders.
Minor Traitors
These are the members of Otherwise Loyal legions usually bringing their entire company or chapter-sized forces with them. After the Treachery, some joined either Luther's Imperium Undivided, or Lorgar's Black Legion. There are also others who have remained rogue entities. While arguably minor players, the skill and power of these individuals should not be under-estimated.
Nassir Amit
Captain of the 5th company of Blood Angels, Nassir Amit had already had many disagreements with both his genefather and the Emperor of Mankind. When Sanguinius ordered him in sentinel duty on Nikea he came into contact with Horus Aximand, with whom he commiserated the current status of the Great Crusade. It really should not have come as a surprise to anyone when he and the Flesh Tearers abandoned Sangunius in "The Little Horus Hersey."
Relevant Quote from the lore. "But I was not", Amit replied. "I was commanded to stand sentinel at this chamber. And perhaps with good purpose. At first I thought it was
Nassir Amit joined with Lorgar in his war against The Primarchs, but out of impatience attacks Sanguinius before he descends to the planet. He and his followers are forced to flee, while Nasir ascends as a Demon Prince of Khorne.
Ptolemy Euergetes
The Great Librarius Ptolemy was captured during the betrayal of Calth and made host for a Slaaneshi Demonette known as "Eugetes" or "The Flutist." Leads a faction of Corrupted Ultramarines.
Tso Zhahaal
As the "War of Knives' ' between Curze and Corax persisted, Tso Zhahaal, already despised by the rest of the Night Lords, abandoned Curze, having seen visions of the ultimate triumph of Tzeentch.
Autek Mor
Already sidelined by his genefather for his barbarity, the Morigal Clan formed an alliance of dissent with several of those Iron Hands who were dissatisfied with the ideals of Ferrus Manus. They were the last to join Luther's Imperium undivided.
"Loyalists"
There were some who claimed to be fighting under the name of The Emperor, but for all intents and purposes were independent factions, pursuing their own ends. Some were outright disobedient, others simply incapably of contacting the rest of the loyalists.
Corvus Corax
Attacked by Konrad Curze and having his planet of Deliverance destroyed, Corax has chased Curze in and out of the Warp for centuries. Ignoring calls from traitor and loyalist alike in his single-minded quest for vengeance.
Konrad Curze
Was deceived by Lorgar into attacking the Ultramarines, angered in being made a pawn in Lorgar's game, and unsure of who to trust, The Night Lords were a rogue entity in the Heresy and well after playing a game of cat and mouse with Corax's Raven Guard.
Servants of the Thirteen (Vigil Opertii/ The Emperor's Scythes)
Forced deep undercover following The conquest of Ultramar, Gulliman's Organizations masqueraded for centuries as a Chaos Cult of Slaanesh, undermining Luther's Imperium from within. They had little to no contact with the Exiled Ultramarines, or the forces of Alicia Ordinata's Movement, and as such they were little more than rogue actors during the heresy. The unspeakable and horrific acts they had to perform against their fellow Ultramari and against loyalists for the sake of maintaining their cover haunt them even into the 41st millennia.
Kharn the Betrayed
Angron abandoning the Butcher's Nails he had forced his sons to take was a step too far for Kharn. And Angron's cowardly bombardment of World Eaters besieging macragge was the final straw. Kharn and a few World Eaters refused to join Angron's new "father" going their own path. They claim they are still loyal to the Imperium, but The Imperium is unable to reach them.
Kelbor-Hal's Mars
The Crafty Fabricator General of Mars, Kelbor-Hal, barely hiding his sympathies toward chaos has declared himself neutral in the great conflict. As a neutral ally, he arguably did more damage to the Loyalist cause than if he had openly joined the traitors, with his regime harassing loyalist supply efforts and severely limiting the imperial war machine. This diplomatic game was not without consequence, as his inaction is causing a schism in the Forgeworlds.
Khornates
Unlike Tzeentch, who played his characteristic double game, Khorne openly disowned the project of Chaos Undivided, furious and what he saw as disrespect and scheming on the part of the other Chaos Gods. Khorne instead decided to take advantage of the Heresy to pursue his own ends of fear and carnage.
The Space Wolves
During the Siege of Prospero, the Space wolves turned on Leman Russ, giving in to their hatred of warp sorcery and becoming feral chaos wolves. Being feral, the Space Wolves don't have an actual allegiance, though their bloodshed and chaos pleases Khorne.
Mortarion
Khorne, furious that Alpharius, Erebus, and Lortara worked to steal Angron for Nurgle, has moved away from the Other gods, instead choosing to pursue his own ends. Mortarion, who learned of Typhus' efforts to bring him to Nurgle, and harrassed by Konrad Curze and Fulgrim, has given into his silent rage.
Tzeentch
In what Lorgar probably should have foresaw, were it not for his own blind Zealotry, The Lord of Schemers had schemes of his own, using Lorgar's Alliance as a way to upend the Great Game in his favor.
The Kabal
Unbeknownst to the Perpetuals who form it, the Kabbal is completely beholden to the whims of Tzeentch, and are pawns to his schemes.
The Alpha Legion
Alpharius learned early of the great handiwork of Tzeentch, and pledged his loyalty to the master of Schemers. He and his twinned soul "Omegon" have been playing a tripple game, being a part of Both Lorgar and Imperial inner circles, manipulating both to make the galaxy witness the glory of the Changer of Ways.
Bel'akor
Belakor refused to bend the knee to Lorgar, and laughed at his pathetic ideals. The original prince of Chaos Undivided is at war with both Lorgar and The Imperium alike, eager to reclaim his favor from the Human Cleric.