The True Sons are a large splinter warband of Chaos Space Marines that formerly belonged to the Black Legion and are dedicated to Chaos Undivided. Following the death of their Primarch Horus at the hand of the Emperor during the final battle of the Horus Heresy and the XVIth Legion's subsequent flight from Terra into the Eye of Terror, Horus' Legion continued to worship him as a god. They bowed down before his newly-raised tomb on the Daemon World of Maeleum where the XVIth Legion found its first refuge and made daily oaths of loyalty. The warband known as the True Sons never broke with these traditions, even after the destruction of the Primarch's body in the wake of the Traitor Legions' internecine Slave Wars. Covering their armour in the profane symbol known as the Eye of Horus, the True Sons build effigies of Horus wherever they go, bowing down to them before symbolically burning them.
In the closing days of the Horus Heresy, the Traitor Legions retreated before the Loyalists, their Warmaster Horus slain by the Emperor of Mankind and their dreams of galactic conquest denied. With their former brothers in close pursuit, the Traitors fled toward the corrupting embrace of Chaos and into the Eye of Terror. Among these Traitors were the fragmented survivors of the Sons of Horus and their greatest surviving warlord: First Captain Ezykyle Abaddon. While other Traitor Legions lost themselves in the madness and excess of the Warp, Abaddon never forgot the defeat of Horus and the debt of vengeance he owed the Emperor. On Maeleum, a graveyard world of steel and rust, the Sons of Horus raised a fortress. Surrounded by living darkness and the bones of dead Warp-dragons, it was fashioned from the wrecks of decaying vessels long lost to the Warp, with each spire and tower made from the jagged prow of an ancient voidship. The Legion interred Horus' body within a great tomb, where many fell into worship of their fallen demigod.
Horus' body hung suspended in a spiralling chamber of bone-white stone, bathed in flickering golden light, his perfect form looking down upon his sons. Reaching up as far as the eye could see, the deeds of Horus were carved into the arching crypt walls, each one depicting a great battle or glorious victory. Each day, the worshippers would gather in the shadow of their Primarch and offer up their oaths anew, unable and unwilling to find a new leader. With their Primarch dead and their Legion on the verge of extinction, the Sons of Horus stagnated. Like many of the other Traitor Legions, the Sons of Horus suffered from incessant daemonic attacks during the early years within the Eye of Terror. There was a never-ending supply of daemons to fight off, and many fell to the uncontrollable influence of Chaos, losing their mind and bodies to possession by these insidious Warp entities.
Some within the Legion argued that the Sons of Horus should offer their allegiance to a single power rather than deal with the daemons of many gods. Most, however, warned that the XVIth Legion should never bow to an outside power again; they remembered too well the yoke that the Emperor had placed around the neck of their Legion, and were wary of letting another master hold such power over them again. The martial pride of the XVIth Legion also meant that they would never completely accept a master who did not come from their own ranks, be it the Emperor of Mankind or even a God of Chaos.
The Slave Wars
The wars between the other Legions who had sided with Horus during the Horus Heresy, known as the Slave Wars, raged across the Eye of Terror even as the Sons of Horus ignored the events happening around them, and continued to raise their fortress ever higher, worshipping the corpse of their Primarch. The Sons of Horus had remained largely apart from these conflicts; however, jealous eyes now turned their way. Traitorous forces gathered against them and conspired to rob them of the remains of Horus to further vile and selfish ambitions. The Primarch Horus' body, with its potent genetic information and biological secrets, was a great prize indeed. In a sudden assault, the remnants of the debased Emperor's Children, having grown vastly in power after firmly cementing their terrible pacts with Slaanesh, easily smashed their way through the defences of Maeleum and into the central chambers of the Sons of Horus' stronghold. They stole the body of the slain Primarch from the heart of its tomb and spirited it away, some say with the purpose of cloning it in order to create a new and still greater Warmaster of Chaos.
Rise of Abaddon
Their fortress in ruins and their Legion decimated, the Sons of Horus stood on the brink of vanishing forever from the galaxy and fading into cursed memory. The XVIth Legion devolved into in-fighting amongst themselves, giving in to dark despair or uncontrolled rage. The divisions between the Legion's captains turned into bitter bloodshed and murder, as order completely collapsed. The salvation of the Sons of Horus came when one of its greatest captains, Ezekyle Abaddon, returned from his Dark Pilgrimage in time to watch the battle from afar. It was in that moment that he saw, with cold clarity, that it was Horus' failure that had led the Legion here, to them tearing each other apart in the blood-soaked ruins of Maeleum. Finally, sickened by how far the Legion had fallen, he stalked through the ruins hunting down his fellow captains, cooling his rage with their final screams. In the end, Abaddon alone remained of the Legion's leaders, demanding obedience from his brothers.
Some saw Abaddon as Horus' rightful successor and fell at his feet willingly, while others recognised his raw strength and bowed their heads to his might. A few turned their back on Abadodn, and were either cut down by their brothers or managed to escape into the Warp. With his Legion brought to heel, Abaddon turned his attention to the clones of Horus; he commanded his warriors to extinguish all trace of their former Primarch and free themselves from his shadow. He then personally led an attack on the Emperor's Children that destroyed the body of the Primarch Horus and all its clones, and in so doing, ushered in a new age for the XVIth Legion. The Traitors changed their name once more, this time in reference to the fact that their armour was now adorned black in mourning for the loss of Horus, calling themselves the Black Legion. Through his actions, Abaddon re-invigorated the Legion, reviving the old notion that none could stand in their way and that they would one day inherit the galaxy itself.
In the long, dark decades after the death of their Primarch, the Sons of Horus fragmented. When Ezekyle Abaddon proclaimed himself the new master of the XVIth Legion in the ruins of Maeleum, there were those who turned their back on the Despoiler. These Legionaries either remained true to the memory of Horus or forsook all masters, before vanishing into the depths of the Eye of Terror. Most would disappear from history forever, but some returned to challenge the Black Legion, or in time rejoin its ranks. They are known to the Black Legion as the Thrice-Cursed Traitors. The True Sons are one such warband; they have never broken with the traditions of worship of their revered Primarch. Covering their armour in the symbol of the Eye of Horus, the True Sons build effigies of Horus wherever they go, bowing down to them before symbolically burning them.
The True Sons still wore the dark and murky verdigris green-black colours of their former Legion. Most notably, they covered their battle plate in the sacred symbol of the Eye of Horus. Their armour also still displayed the Cthonian gang-sigils graven into their armour that recorded their notable kills and deeds.
The True Sons continued to display the Eye of Horus in tribute to their fallen Primarch.
- Black Legion - A Codex: Chaos Space Marines Supplement (Digital Edition), pp. 12-14, 63