With the last hope of unity among the damned lost, the Eye of Terror became the battleground of the Traitor Legions as they made war against each other. On Sortiarius, Lorgar and Magnus met, and forged the Crimson Accords : a set of treaties by which the alliance between the Thousand Sons and the Word Bearers, begun in the failed attempt to kill Horus at Zu'lasa, was solidified. As part of this alliance, each Legion sent an ambassador to the other's homeworld. Lorgar's envoy to the Planet of the Sorcerers was the Dark Apostle Angra Kalar, while Magnus dispatched a Sorcerer named Ptolemeraz to Sicarius. The two of them were warriors who had distinguished themselves during the rebellion, but had only recently been elevated to positions of power – Angra Kalar after the Word Bearers are reformed at Sicarius, Ptolemeraz after the Rubric had decimated his Legion.
Both emissaries were allowed only a token honor guard : the Dark Apostle chose a few of his most loyal disciples, while the Thousand Son was accompanied by a handful of silent, soul-dead Rubricae. The two quickly built strongholds on their appointed daemon worlds, and through the common sorcery of Lorgar and Magnus, a permanent portal was opened between these locations, allowing warriors from both Legions to travel back and forth between the two planets instantly. Soon, the embassies became a nexus of activities, as warriors sought to renew old friendships with their cousins, or forge new ones. Angra Kalar and Ptolemeraz especially worked together, and the fruits of their collaboration made their embassies unsettling, dangerous places, even for the Eye.
Dark lore flowed between the two Legions as well. The libraries of Sortiarius, containing what had been salvaged from Prospero as well as what the Thousand Sons had been able to plunder during the Heresy, had plenty to offer to the Word Bearers, whose mastery of rituals and practice with the infernal was greater than that of the Fifteenth Legion in some areas. The Seventeenth Legion also had access to much greater resources than the Thousand Sons, whose numbers, always smaller than any other Legion, had been decimated by the Burning of Prospero and Ahriman's ill-fated Rubric.
Across the two daemon worlds, Sorcerers and Dark Apostles summoned and bound Neverborn into their service, gathering an infernal legion of a size rarely seen before in galactic history, and never under mortal command. Thousands of daemons were chained to the will of the Legionaries, to be used as shock troops by the Crimson Accords. Circles of Astartes worked together and burned the souls of thousands of lesser witches in order to draw out and control more powerful members of the infernal choirs, from the Greater Daemons of the Primordial Truth to the Daemon Princes who had once been mortal.
Meanwhile, in Horus' domain, the World Eaters and Emperor's Children that the Warmaster had rallied at Skalathrax had broken apart. The conflict between Fulgrim and Angron at Zu'lasa had echoed through the blood of their sons, and they had broken their vows of service to Horus rather than serve alongside those they despised.
The eight warlords Horus had chosen to govern the Twelfth Legion led their brothers away from the Sixteenth Legion's territory. Knowing that the Legion couldn't hope to remain united for long before turning on itself, they divided the forces under their command into eight lesser hosts, giving individual leaders the choice of which of the eight they would follow. After that, they spread across the Eye, seeking bloodshed to appease the biting of the Butcher's Nails. The Eye of Terror provided plenty of battlegrounds for them to join. The daemon worlds of Khorne, and those forever fought over by the Ruinous Powers, drew them like moths to a flame, often with the same conclusion. But though many sons of Angron perished in these wars, fought only for the sake of fighting, the survivors grew strong, their bodies slowly reforged by the Warp and the favor of the God of War. Some fought on the battlefields of the Legion Wars, either because of ancient debts owed to individuals on one side or the other, or as wild berzerkers, uncaring who they killed so long as the blood flowed. The Word Bearers, thanks to their experience in the Shadow Crusade, were the most able to manipulate the fury of the Twelfth to serve their own end – but even they paid a price in blood sooner or later, which they accepted as the God of War's holy due.
The Emperor's Children, having heard of their Primarch's return and subsequent disappearance, refused to kneel to Horus any longer, their pride demanding that they serve no one but their own gene-sire if they were to serve any Primarch at all. Fully aware that many in the Eye blamed the Third Legion's raids upon Terra's population for the rebellion's failure, the sons of Fulgrim banded together, out of self-preservation rather than brotherhood. They conquered several daemon worlds and, using the slaves filling the holds of their ships, built vast, decadent cities and palaces there. The glory of Slaanesh flowed through them, inspiring them in equal measure to the ruination it had already visited upon them. They crafted horrible wonders and learned many secrets, which they used to further their own ends. In immense, gilded arenas, the champions of the Legion duelled against each other as well as against creatures and monsters coming from all across the Eye. In these kingdoms, the daemons of Slaanesh walked openly, devouring the Emperor's Children's mortal slaves after granting them one brief, ecstatic glimpse of the Dark Prince's perfection. All that mattered to the Emperor's Children was their own pursuit of excess : their only part in the Legion Wars was when some warlord or another gathered an army to raid the lesser outposts of other Legions in order to capture more slaves for their pits.
Only the Night Lords remained allied with the Sons of Horus, and though Curze was in command of the largest Astartes force in the Eye, the Eighth Legion had even fewer Sorcerers than the Sons of Horus. The Horusians, as the alliance of warriors loyal to the Warmaster came to be known, were at a decisive disadvantage where sorcery was concerned, and in the Eye of Terror, where reality itself was endlessly reshaped by the souls walking it, that was a dangerous weakness.
Mere months after the Conclave, the Crimson Accords attacked the Horusians. At the head of the onslaught were the daemonic hordes, unleashed upon the worlds of the Warmaster's dominion with barely any care given to strategy. The Neverborn crashed against the Horusian defences like a tide, leaving the defenders vulnerable to the precision strikes of the Word Bearers and the sorcery of the Thousand Sons. The Sons of Horus fought with all their might, reinforced by troops sent from Kerlazium by Curze – though controlling the more degenerate Night Lords was a challenge in itself.
But even then, slowly, the Horusians were losing. Outpost after outpost, stronghold after stronghold, world after world were falling to the forces of the Crimson Accords. Whole fortresses were lost to single Sorcerers, who used their powers to get into the minds of the defenders and turn them against one another or cause catastrophic accidents. Those few outposts which had a Sorcerer of their own to hold the psychic intrusions at bay and ward the walls from daemons were besieged by Word Bearers War Hosts, each led by a Dark Apostle – who was often a Sorcerer himself.
Seeing which way the wind was blowing, the other powers of the Eye aligned themselves with the Crimson Accords. Dark Mechanicum forge-worlds, Fallen Knight households, renegade Navy fleets and Army Regiments pledged their support to the two Daemon Primarchs. Many did so out of a desire to be on the side of the Legion Wars' victor, while others sought revenge against Horus and his sons, believing that it was them who had cost the rebels the war.
Maeleum itself did not come under attack : whether the forces of the Crimson Accords feared Horus' power, or wanted to make the Warmaster watch his dominion burn before going for the kill, they left the Sixteenth Legion's new homeworld alone. They didn't need to hurry : if nothing changed, their victory was inevitable. But Horus had never been one to let such challenges go unanswered, and he sought a mean to turn the tides of the Legion Wars.
The plain and simple truth was that the Horusians needed more Sorcerers. Training Librarians and mortal witches was a stop-gap at best, a desperate move at worst. But there was one group of Sorcerers who had good reason to stand against the alliance of Magnus and Lorgar : the remnants of Ahriman's Cabal. Ahriman had been banished from Sortiarius for casting the Rubric, the spell that had all but murdered the Thousand Sons. His cabal had been banished along with him, and they had fled from the wrath of Magnus and their own brothers.
With the rise of the Crimson Accords, these exiled were being hunted. Magnus may have sentenced them to being cast out, but those who had watched their brothers turn to dust before their eyes craved revenge. As a result, they had gone to ground, hiding in the most unlikely places or finding refuge among the warbands of other Legions – several had been recruited by the Emperor's Children, who enjoyed their misery and their skills at daemon summoning.
Never one to use half-mesures, Horus summoned a powerful daemon of Tzeentch and bound it to reveal to him the location of Ahriman himself. When the daemon failed to answer, Horus destroyed its physical form and sent its spirit shrieking back into the Warp, before summoning the next one. The thirteenth daemon thus summoned managed to bargain for its existence : though it did not know where Ahriman had fled to (for the exiled First Captain had covered his tracks well), it knew where Horus could start to look. And so Horus left Maeleum, giving command of the Horusian war effort to his Mournival.
How long the Warmaster was gone for is difficult to tell : on some battlefields, decades passed without his advice and aid, while he reappeared on Maeleum only nine weeks after his departure. As the Primarch walked out of a rift in reality and into his throneroom, at his side stood Ahzek Ahriman.
'You were the one who sent the Wolves to kill us all. Why would I ever follow you ?'
'What price would convince you, Ahzek ?'
The Exile laughed bitterly.
'Can you restore Prospero ? Can you unmake the Rubric ? Can you remove the curse of the flesh-change without it killing my Legion ?'
Horus smiled …
Whatever Horus offered to or threatened Ahriman with, the Exile accepted the Warmaster's offer. His first task was to journey across the Eye and make contact with the other members of his cabal, scattered throughout the storm after their banishment from Sortiarius. One by one, Ahriman found them, and brought them into the fold. These Cabalites then followed him to Maeleum, where they began to work on countering the sorcerous advantage of the Crimson Accords.
Many Sons of Horus were in awe of their Primarch's power, and had been since he had claimed it on Molech. They remembered the awesome displays of his might that he had performed during the rebellion, and now, with enemies of the Legion pressing in, they sought to emulate him by taking in the power of the Warp within themselves. With the help of the Cabalites, thousands of them walked into the fires of the Empyrean, and those who emerged were transformed into Secondborn, granted increased strength and resilience by the daemons now sharing their flesh. Despite the losses incurred in such transformations, these Possessed Marines proved a boon to the Sons of Horus, capable of fighting the daemonic hordes of the Crimson Accords on equal ground.
Between the recruitment of Ahriman's Cabalites and this influx of Secondborn, the imminent defeat that had loomed over the Horusians was banished. But the advantage in the Legion Wars was still very much on Lorgar's and Magnus' side.