Maeleum

Maeleum is a Daemon World located within the Warp-realspace interface known as the Eye of Terror in the Segmentum Obscurus. For a time, Maeleus 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.

History
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 in 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 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 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. The current disposition of Maeleum remains unknown.