News eventually reached Maeleum that forces from other Legions had been discovered. Elements of the Third and Twelfth Legions had been sighted by the exploring warbands, wandering the outer regions of the Eye. It was difficult to get precise numbers, but the scouts were confident that thousands of Legionaries had arrived in the Eye, scattered by the currents of the Warp.
Immediately, Horus left Maeleum aboard the Vengeful Spirit, seeking to bring these wandering warriors under his banner. The growing domain of the Sons of Horus was left in the care of Abaddon, Horus' most trusted son. It would later transpire that the Warmaster had departed not a moment too soon, for when he found the Emperor's Children and the World Eaters, they were on the verge of doing what the Imperium had failed to accomplish : destroying each other for good.
After arriving in the Eye, the forces of both Legions had slowly coalesced together, and had finally met in a system with a single world, one afflicted by violent nocturnal storms that not even Space Marines could survive. That world had been named Skalathrax by Khârn, Eighth Captain of the Twelfth Legion and Equerry to Angron, who was said to have died on Terra only to be resurrected by the God of War as the World Eaters left reality and entered the Eye of Terror.
The reunion of the two Legions had been extremely tense, for though few warriors understood it yet, the Emperor's Children and the World Eaters belonged to opposing Ruinous Powers, and the contempt that had always existed between them was now simmering into hate. The Lord-Commander Eidolon, who led the Emperor's Children in Fulgrim's absence, had called for parley, and Khârn had accepted, proposing that the leaders of both hosts meet on the planet's surface.
Eidolon had intended to send an emissary rather than meet Khârn in person. However, the Eighth Captain saw through his intent, and made the Lord-Commander's presence a condition to the discussion. Amused that the brute had been able to anticipate him, Eidolon had accepted, though he had asked in return that Khârn only bring with him a far smaller escort than the one Eidolon would travel to Skalathrax with. Khârn, seemingly uncaring of the disrespect of the offer, had agreed.
'This is madness. You cannot mean to do this, Khârn. I have fifty warriors at my side ! What do you have ? A handful of blood-crazed barbarians ?'
'I have my axe. I have my fists. I have the Nails. That is more than enough.'
From the meeting of Khârn and Eidolon
The meeting degenerated into a slaughter, with Eidolon and Khârn being the only ones to walk away from it alive. This was no small feat for the World Eaters, given that they had been outnumbered ten to one at the meeting, and that the Lord-Commander had brought one of his pet daemons with him, a fiend known as Hedonarch and quite highly placed in the Dark Prince's choirs. Yet Khârn, wielding the axe Gorechild, had slaughtered the guards and entourage of Eidolon before forcing the Lord-Commander to flee for his life (though of course Eidolon told a different story to his men). The war began while the two lords were still in their transports, as the fleets activated the weapon arrays that had been kept dormant during the meeting as a gesture of good faith.
With the crews of both Legions still adapting their training to the impossible physics of the Eye of Terror, the war had to be taken planetside. Thousands of Space Marines, hundreds of thousands of mortals, and the accompanying tanks and artillery were brought on Skalathrax's surface. The Emperor's Children brought horrors of flesh from their unholy laboratories, and hordes of debased, mind-blasted cultists whose veins contained more drugs than blood. The sands of Skalathrax ran red, until the sun began to set, and the storms came, the temperature plummeting far below the point of freezing water. Only then did the battle end, as the forces of both Legions retreated, leaving the corpses of the dead to freeze where they had fallen.
For six days, the pattern repeated itself. The Legions would fight from dawn till dusk, and withdraw once Skalathrax itself made further conflict impossible. Then, on the seventh day, Horus arrived. The Vengeful Spirit and her escorting fleet emerged at the system's edge, catching the fleets of the two Legions by surprise. The Warmaster saw what was happening as reports flooded in from the fleet's auspexes. He immediately understood what had occurred, and he was not pleased.
"Enough.
I will not allow this madness to continue.
We are banished from my father's kingdom, cast out by those we called brothers, because we sought to free Mankind from the manipulations of a tyrant. All of us joined this cause for our own reasons, and that is well, for freedom is what we fought for.
But this … this is a travesty, and I will not stand for it.
This ends. Now."
From the proclamation of Horus at the Battle of Skalathrax
The sheer presence of Horus, and the might of the fleet that accompanied him, were enough to force a truce between the warring factions. Companies of Sons of Horus descended upon the battlefields of Skalathrax as dusk fell and the cold forced each side to retreat to their shelters. With the hand of the Warmaster guiding them, they effectively took hostage the forces on the daemon world, forcing the commanders of both armies to the negotiation table.
Both Khârn and Eidolon were summoned to the Vengeful Spirit, without any honor guard this time. They came to the strategium through separate corridors, escorted by black-clad Terminators of the Justaerin, and faced Horus' judgement in the same chamber where the Siege of Terra had been planned in the last days of the rebellion. Horus expressed his displeasure, and demanded that they explain themselves. The Lord-Commander waxed on, dramatically retelling how Khârn had breached the truce and attacked him and his escort, slaughtering them all with unthinking brutality – even turning on his own brothers once Eidolon had escaped. The son of Fulgrim delected in the situation – as a true devotee of the Dark Prince, every sensation was to be savored, and there were few feelings rarer than the wrath of the Warmaster. Eventually, his tale came to an end, and Horus asked Khârn to tell his side of the story, that he may weigh their testimonies with the evidence his Legion's investigators had already gathered. For several seconds, Khârn remained silent.
Then he drew Gorechild, and leapt at Horus, the weapon raised, its dragon's teeth roaring with a thirst for blood it shared with its wielder. He moved with speed beyond mere transhuman abilities, but was still no match for Horus' reflexes. The Warmaster struck with Worldbreaker, the mace catching the World Eater mid-jump and sending him flying across the room. Most Astartes would have been dead, but Khârn rose, facing Horus in his anger. For a few seconds, the two of them stared at one another, with Eidolon witnessing the scene with baited breath. Then, Khârn turned and fled, killing every Son of Horus in his way before taking a gunship to the closest Twelfth Legion vessel and forcing its captain to depart Skalathrax while the confusion was still reigning.
In hindsight, it became clear that Khârn's flight had been facilitated by one of the Ruinous Powers – no matter how deadly the World Eater champion might be, escaping the Vengeful Spirit alone was something even Malcador's hand-picked agents hadn't been able to do during the rebellion. But, through a series of unlikely coincidences and the sheer brutality of the World Eater, Khârn managed to escape the Warmaster's wrath, departing through the storm toward some unknown destination.
For this, the Eighth Captain of the World Eaters was named the Betrayer, a name that would follow him everywhere he went. But Khârn still had the favor of the Blood God, and many would seek to manipulate him to kill their enemies for them in the years to come – though more often than not, they ended up dead by his hand as well. Some would claim that Khârn had actually died on Terra, at the hands of the defenders of the Imperial Palace, and that his apparent resurrection had been a daemon's trick : nothing more than a disguised possession of a dead body. The motive for such an act varied depending on the teller : some said the daemon wanted to claim the World Eaters as its own, others that it wanted to destroy the Third Legion or the Twelfth, and others still thought it had all been a ploy in order to get close to Horus, just for the chance to strike at him with Gorechild, which was one of the few weapons that could still harm what the Warmaster had become. To most, however, these theories were nothing more than wishful thinking trying to justify a madman's act.
Despite Khârn's escape, the Battle of Skalathrax confirmed that the other rebel Legions were making their way toward the Eye of Terror. The World Eaters and the Emperor's Children both had Primarchs who had shed their last ties to mortality and ascended to daemonhood, and now Angron and Fulgrim were calling their sons to join them in the Eye, where they had been reborn following their dissolutions at Terra. Though the hatred between the two Legions was strong, Horus managed to force them both to follow his command for the time being by offering them the one thing they all desired : a chance to be reunited with their Primarchs. Horus was, after all, the Warmaster of Chaos, chosen by the Ruinous Powers to bring down the False Emperor.
Grudgingly, and with the Vengeful Spirit between them, the World Eaters and Emperor's Children accompanied Horus back to the dominion of the Sixteenth Legion. Skalathrax was abandoned, left to the ghosts of the thousands who had died for nothing on its surface – a testament to the self-destructive nature of the Dark Gods that would stand for thousands of years to come.