Lorgar's withdrawal and the fracturing of the Seventeenth Legion's leadership cemented the turning of the tides in the Legion Wars. Under the Warmaster's guidance, the Horusians launched an aggressive campaign of conquest on the territories of the Crimson Accords. Minor powers that had aligned themselves with Lorgar and Magnus changed sides, paying a heavy price in flesh and resources in order to appease the Warmaster's wrath.
Though Lorgar had abandoned the war, Magnus had not, and the Crimson King was an enemy to be feared. His attempts at drawing Lorgar out of his revelation-induced withdrawal failed, leaving the Cyclops in sole control of those forces still loyal to the spirit of the Crimson Accords. From his Tower on Sortiarius, Magnus performed many great feats of sorcery whose effects reached across the Eye and granted infernal assistance to his armies.
On Maeleum, Horus received reports of these rituals' effects : the earth opening beneath rolling tanks, rains of acid falling from clear skies upon mortal armies, Warp eruptions that swallowed whole companies of Astartes and spat out twisted monstrosities. The Warmaster doubted that his brother could keep up such things for long : there was a cost for all sorcery, and even a Daemon Primarch could not escape the backlash forever. But he could not (he would not) simply wait to outlast the storm of Magnus' fury, and waste the lives of his warriors to a war that was ultimately a test at best, and a distraction at worst.
Instead, Horus called for Ahriman, and commanded him to go to the Planet of the Sorcerers and bring an end to the Legion Wars once and for all. The assaults on enemy daemon worlds stopped, and the Warmaster recalled the forces thus freed to the Sixteenth Legion's homeworld, before placing them under Ahriman's command. It was a mighty fleet indeed, counting hundreds of ships and nearly twenty thousand Legionaries from several different Legions. Some of the warlords resented being placed under the leadership of one who bore the same blood as their enemies, but they were silenced by the presence of none other than Ezekyle Abaddon at Ahriman's side. Whether the First Captain was given the assignment to keep the fleet under control, or as insurance against any last-minute treachery on the Exile's part, only he and the Warmaster knew.
At the head of this fleet was the Vengeful Spirit herself, which had already played a key part in the Legion Wars. The prowess of the Gloriana-class battleship had been respected by the Traitor Legions before : now they feared her guns, which seemed to hunger for their blood. The flagship and a handful of escorts were the whole of the Sixteenth Legion presence amidst the fleet : the rest were Night Lords, Dark Angels (though Vortigern was no part of the force, being left to direct the defense of one of the fronts), and an assortment of warbands from the other, neutral Legions. Ships of the Death Guard had quietly arrived to Maeleum as the fleet gathered, having learned of its purpose through means unknown and seeking to participate in the assault on the homeworld of the Legion of their Dark God's rival.
Once preparations were complete, the fleet left Maeleum on a direct course to Sortiarius. Here, the unique nature of travel through the Eye worked in the Horusians' favor : there were no Warp routes they needed to follow, no need to emerge from the Empyrean at the Mandeville Points of the systems they had to cross. With the Cabalites scattered among the fleet and linked to one another telepathically, the fleet could brave the madness of the storms and sail straight for the Planet of the Sorcerers.
Of course, the fleet's Warp displacement guaranteed that the Crimson King knew about it as soon as it left, if not before. But Horus was gambling that one spear thrust, aimed at the heart of enemy territory, could crash through whatever defenses Magnus could raise in its path fast enough. He aimed to end the war faster than the grinding warfare that would otherwise be needed to reduce the influence of the Crimson Accords forces to the point where they stopped being a threat (something that, given the capabilities of some Thousand Sons and Dark Apostles, could very well require the full purge of two Legions which were supposed to be on his side in the Long War). It was a tactic that the Sons of Horus had mastered during the Great Crusade, and often used during the rebellion, though it had ultimately failed them at Terra.
The Horusian fleet slammed into Sortiarius' defenses the moment they emerged from the storm and entered the pocket of more-or-less stable reality around the Planet of the Sorcerers, where the laws of physics were only carelessly broken rather than chewed on, digested and spat back out, transformed into the whimsical nightmares of mad godlings.
Magnus had called all the forces he could to help defend Sortiarius, and many had answered the call. A gauntlet of ships, defense stations and other, stranger things awaited Ahriman and Abaddon. The First Captain of the Sons of Horus took command of the battle, while the Exile went into the depths of the Vengeful Spirit and began the final step of the plan he had begun to conceive when Horus had given him this mission. Using the connection that lingered between him, Sortiarius and Magnus (leftover from the great spell that had transported the Thousand Sons there from Prospero), he opened a rift between the flagship and the top of the Tower of the Cyclops and walked through, armed with his Black Staff and an athame dagger.
The Crimson King was there, watching the battle unfold in the heavens with his burning eye. What happened next is unclear : Ahzek could not have hoped to defeat Magnus alone, not even with the Crimson King's focus being on the war above. But, somehow, perhaps by channelling the sorcerous backlash that had accumulated through Magnus' reckless support of the Crimson Accords forces, the Exile managed to bring his Primarch to his knees.
Whatever the Exile intended to do next was interrupted by the sudden arrival of someone Ahriman knew of old. Iskandar Khayon, Captain of the Thousand Sons and once a member of Ahriman's Cabal, before turning on them and trying to stop the Rubric when it had started to go wrong. The red of his armor was as vivid as Ahriman's azure, and the two of them clashed atop the Tower, before the subsumed form of their Daemon Primarch.
"Was it not enough to kill us, Ahzek ?! You had to betray us to Horus as well ?!"
"What I do, I do for us all. For Tizca. For Prospero."
"Then you are foolish as well as damned."
"... I am aware, brother."
Exchange between Iskandar Khayon and Ahzek Ahriman
In the end, Ahriman killed Iskandar, shattering his brother's prized axe Saern with the Black Staff before burying the athame that had been meant for Magnus into his hearts. But by then, it was too late. Before the Scarab Guard Terminators flying up the Tower on bound Screamers of Tzeentch could stop him, Ahriman teleported back aboard the Vengeful Spirit, leaving Magnus to recover from the sorcerous blow. Khayon's body was carried away by his bloodward, a black-winged Dark Eldar, who took it back to his ship, the Tlaloc. The vessel left Sortiarius soon after, without answering any hail of the Thousand Sons or their allies.
Though Ahriman's gambit had failed thanks to Khayon's sacrifice, Magnus was forced to recognize that he could no longer hope to win the Legion Wars. As the Horusian fleet finished off the remnants of the gauntlet and prepared for a direct attack on Sortiarius, the Crimson King sent a psychic message. It wasn't addressed to the attackers, nor to his renegade son (though both heard it nonetheless), but to his brother Horus himself.
In essence, the message was a plea for peace and an offering all in one. Magnus knew that Horus sought vengeance over the Imperium more than he desired to destroy the Fifteenth Legion, and in exchange for the withdrawal of the Horusian fleet and a détente of the relationship between the Crimson Accords and the Warmaster's growing empire, he offered the keys to escaping the Eye of Terror.
Horus accepted Magnus' offer. The fleet returned to Maeleum, with an emissary carrying the Crimson King's tribute held aboard the Vengeful Spirit. The envoy was dragged in chains before Horus, and revealed the gift of his Primarch : a set of rituals, designed by the Crimson King. When performed by mortals outside the Eye of Terror, these rituals would open Warp portals between their location and the Eye, effectively enabling the summoning of Astartes beyond the borders of their prison. With this, they could bypass the Cadian Gate, though the rituals would not work for anything bigger than a Terminator - not without significant modifications to accommodate for the increase in scope, at least.
Ahriman and the other Caballing Sorcerers studied the rituals they extracted from the opened mind of the envoy (which was devoid of anything other than his mission, his mind having been wiped clean by Magnus beforehand). They found that the envoy had spoken true, and the rituals could be adapted in order to "resonate" with different daemon worlds. They had also discovered several backdoors which could be used by powerful Sorcerers who knew of them to sabotage the rituals or subvert them, but such treachery had been expected from the Daemon Primarch of Tzeentch (if Magnus had even known they were there).
The Cabalites presented their findings to Horus, who considered his options. After several hours of deliberation and discussion with his warlords, Horus ordered that an Eye-wide cease-fire be enacted between the Horusians and the Crimson Accords. The Legion Wars were ended, though there would always be spots of warfare between individual warbands – it was only the greater conflict that was over. To prevent the Wars from erupting again in the future, Horus sent emissaries of his own to all other Legions, bringing copies of Magnus' rituals. Two ill-favoured sons were sent to the World Eaters and the Death Guard, and only one of them returned alive, but by the end all Traitor Legions had access to the rituals.
But useful as they had the potential to be, the rituals couldn't serve their purpose unless they were in mortal hands outside the Eye of Terror. And so Horus thought on what Moriana had told him, and began to enact his plan for the next phase of the Long War.