Although Abaddon was known as the Chaos Warmaster, compared to the hundreds of thousands of Astartes under Horus when he had been Warmaster, the forces Abaddon could directly command numbered no more than fifty thousand.
They were the last of his foundations, cultivated anew using the gene-seed reserves of the Sons of Horus and gene-seed traded from Fabius Bile, the apothecary of the Third Legion.
Of those fifty thousand, roughly fifteen thousand Space Marines bore Horus's gene-seed, while the rest were Astartes who had voluntarily joined the Black Legion and obeyed Abaddon's commands without reservation.
Setting Abaddon's other qualities aside, he truly possessed extraordinary personal charisma and leadership, enough to gather together once more those Astartes who had been scattered during the Great Heresy, lost their direction, and completely lost their central pillar.
Naturally, the Black Legion could not consist of only fifty thousand men.
Abaddon had also reached agreements and forged alliances with the leaders of those Chaos Astartes warbands. Whenever he needed them, he could issue a summons and command them under a unified deployment.
For the vast majority of the time, the Black Legion did not act as a single body.
On one hand, acting collectively inevitably reduced the scope of their scavenging and plunder. On the other, when gathered together, conflicts could easily erupt between the various Chaos warbands.
After Horus was killed by the Emperor and the rebels were forced to flee into the Eye of Terror, they were no longer allies, and no one could restrain them by force.
Within the resource-starved Eye of Terror, even former battle-brothers might turn their blades against one another over resources and old grudges.
This was also the price they had inevitably paid after embracing Chaos: the essence of Chaos was disorder, and the discipline of Chaos warbands