Maeleum is a Daemon World located within the Warp-realspace interface known as the Eye of Terror in the Segmentum Obscurus. For a time, Maeleum served as the first homeworld of the Sons of Horus Traitor Legion following the immediate end of the Horus Heresy in the early 31st Millennium, though the world was abandoned in the wake of the Eye of Terror Slave Wars.
Using slaves captured from their campaigns against the Imperium during the Heresy, the XVI Legion was able to build a massive fortress on the planet and entombed the corpse of their slain Warmaster Horus within it. However, not long after, the Eye of Terror Slave Wars erupted and the Emperor's Children Legion launched a devastating surprise assault on Maeleum, devastating the Sons of Horus' fortress and stealing Horus' corpse. This was part of an attempt by the corrupted Apothecary Fabius Bile to clone the dead Warmaster of Chaos and restore the fortunes of the Forces of Chaos in the wake of the Imperium's victory.
It was at this time that Abaddon the Despoiler returned to the Sons of Horus to lead his forces to recover Horus' corpse. Following the successful defeat of the Emperor's Children and the shattering of that once-proud Space Marine Legion into gibbering, hedonistic warbands, the Sons of Horus recovered the corpse of their former leader.
Abaddon returned it to Maeleum where he had it destroyed so that no further attempts to clone Horus could ever be attempted. 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 XVI Legion's former moniker of the Sons of Horus.
From then on, they became known only as the Black Legion. The Black Legion 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.
During the preliminary stages of the 1st Black Crusade, the Black Legion gathered their forces and withdrew from much of their territory, leaving only skeleton forces at key outposts and minimizing the patrols of their domain. Every Astartes was vital to the campaign. This left much of their realm unguarded. Thaegus Daravek, Lord of Hosts and Abaddon's most hated rival at that time bombarded Maeleum from orbit, shattered the planet and used those pieces as ammunition, propelled by mass drivers, to attack the Black Legion Fleet.
- Black Legion: A Codex: Chaos Space Marines Supplement (6th Edition) (Digital Edition), pp. 13-18
- Black Legion (Novel) by Aaron Dembski-Bowden