To create splits in the Accord nations and create defections. They might have been expecting the White Court to join them, or Marcone to withdraw, or Ferro.
All they got was the Ghouls, nobody likes the Ghouls. “Bah”, said Corb, “all we got was the Ghouls, do you know how much they need to eat? And the smell!” He said raising a perfumed handkerchief to his toadlike face. “Pure muscle, and no magic”
Marcone was probably a miscalculation, they assumed a mere mortal would fold. The White Court, maybe under Papa Raith, but less likely so under a distracted Lara, unless there is a coup going on in the background at the White Court. The former makes it more likely Thomas is not being used by the Black Council, the latter does.
If as posited they had already split off Drakul this makes sense, building an alliance to take on the mortals. The White Court would be singularly used in undermining mortal leadership with their sexual favours, although to be fair they seem to slip up sexually enough on their own.