The Daughter's Due (2019)

From the introduction:

The Blakros family is among Absalom’s most influential families, having secured considerable riches by smuggling goods between Qadira and Taldor during the centuries-long war between these nations—a war that ended only a few generations ago. The secrets of their success were twofold. First, the Blakroses married off their daughters in exchange for exorbitant bride price payments, growing the family’s wealth and establishing these daughters in positions of influence. Anyone marrying a Blakros daughter was expected to take the Blakros name, further securing the family’s prominence. Second, the Blakros family established a millennium-long contract known as the Penumbral Accords, which granted the family untouchable trade routes through the Shadow Plane. In exchange, the family provided their fetchling partners in the Onyx Alliance, a sinister organization based in Shadow Absalom, with yearly shipments of slaves. Once each decade, the family was also required to surrender the eldest Blakros daughter to the Alliance. Were the family ever to break the deal, the fetchlings promised that every other Blakros daughter would die.