The Legion Wars, known also as the Legion War, the Slave Wars, or sometimes referred to as the Eye of Terror Slave Wars, were waged shortly after the end of the Horus Heresy in the Eye of Terror between the various Traitor Legions of Heretic Astartes in the early 31st Millennium. This conflict marked the final collapse of unity between the Chaos-corrupted Space Marine Legions that had fought unified under the banner of Horus Lupercal during the Horus Heresy.
It also marked the transformation of the Sons of Horus into the Black Legion led by Horus' ultimate successor as the Warmaster of Chaos, Abaddon the Despoiler, the former First Captain of the Sons of Horus. Many of the events of this conflict remain unclear or distorted to Imperial chroniclers due to the difficulty of gaining information about the actions of the Traitor Legions within a Warp rift like the Eye of Terror.
The first major war between the most powerful mortal servants of the Dark Gods following the Horus Heresy was started by the Emperor's Children Legion, that Traitor Legion dedicated to the service of Slaanesh, whose excesses grew more wanton and uncontrollable in the days after the Heresy. As the supply of slaves the III Legion had acquired on Terra and other Imperial worlds as they fled to the Eye was exhausted, the Emperor's Children began to assault the positions of the other Traitor Legions within the Eye of Terror, plundering their own slave stocks for use in satisfying their perverse, hedonistic whims. It is for this reason that these campaigns are collectively remembered as the Slave Wars. One of the actions known to the Imperium that occurred during these conflicts was the Battle of Skalathrax between the Emperor's Children and the World Eaters that saw the World Eaters finally shattered as a unified Space Marine Legion.
The Emperor's Children also destroyed the fortress of the Sons of Horus Legion on the Daemon World of Maeleum, leading to a failed attempt by the corrupt Emperor's Children Apothecary Fabius Bile to clone Horus and provide a new leader for the Forces of Chaos. Abaddon the Despoiler led the Sons of Horus to reclaim Horus' corpse, and destroy the primary fortress of the Emperor's Children. Abaddon returned it to Maeleum where he had it destroyed so that no further attempts to clone Horus could ever be attempted. Following this triumph, Abaddon declared himself Horus' successor as the Warmaster of Chaos and the master of the XVIth Legion. At the same time, he came to the epiphany that Horus was dead because Horus had been a weak fool, unable to complete his task of slaying the Emperor and taking control of the galaxy in the name of the Dark Gods. Abaddon swore that he would succeed where Horus had failed in overthrowing the "Corpse-Emperor" and proclaimed himself the new Warmaster of Chaos. He had the Sons of Horus repaint their viridian Power Armour black, the colour of mourning and of vengeance, and cast off the XVIth Legion's former moniker of the Sons of Horus. From then on, they became known as the Black Legion. The Black Legion then abandoned the ruins of the Sons of Horus' fortress and left Maeleum behind, choosing to become a fleet-based Legion with scattered holdings all across the Eye of Terror.
Though long, terrible, and bloody, in the end the dwindling Emperor's Children Legion faced their inevitable defeat at the hands of their fellow Traitor Legions. The Emperor's Children, like the World Eaters before them, were finally shattered as an organised Space Marine Legion and broke up into hundreds of disparate and often opposed Chaos Space Marine warbands. It is said by the adversaries of those who serve Slaanesh that during the Slave Wars the Daemon Primarch Fulgrim was himself killed by Abaddon and the Black Legion, but the Emperor's Children counter that he was given lordship by Slaanesh over an unknown Daemon World of unending pleasure, its location unknown even to many of their own ranks.
- Black Legion - A Codex: Chaos Space Marines Supplement (Digital Edition), pp. 13-18
- Index Astartes I, "Children of the Emperor", pg. 29
- Index Astartes III, pp. 34, 40
- Index Astartes IV, "Sons of Horus - The Black Legion Space Marine Legion"
- The Talon of Horus (Novel) by Aaron Dembski-Bowden