The Gilded Cage Guild, based in Hell 71 and ruled by Guild Master Supreme Praman Nand, deals with slavery, drinks, and other exotic vices. It is one of the most powerful slaver guilds in the Outer Rim of Throne.
The devil known as Yabalchoath was once the co-equal boss of the Gilded Cage Guild with her husband Praman Nand, until her excesses led to her downfall and banishment. When Pramand Nand was finally able to summon his spouse back to Throne, she incarnated as a lesser blue devil. This being now known as Cio returned to work in the Guild - but at a much lesser status given her loss of incarnated power and perhaps also due to her lack of motivation to return wholeheartedly to the path of power and domination.
As the Gilded Cage Guild ( among others ) provides services for the Demiurges, these services of obtaining, selling, buying, and transporting slaves were conducted under their auspices -as seen during the conquest of Rayuba. King of Swords 4-44
The practice of slavery continues into the present under the rule of the surviving seven "Black Kings". Slaves are acquired from conquered worlds and transported elsewhere to markets and buyers. While the practice of slavery is abhorrent, and against the laws that the Angels seek to enforce - the fact is simply that the Guilds ( and their patrons, the Seven surviving Demiurges ) are far more powerful than what remains of the Concordant Harmonies.
Praman Nand was well aware of this power imbalance when he taunted 82 White Chain during the peacekeeper's visit to Hell 71 with Allison. KSBD 2-35. Praman Nand mistakenly assumed that 82 White Chain was there to try to free all of Praman's slaves, but knew full well that would be impossible - even for one very dedicated, and very accomplished angel.