I’m not even sure if a feral vampire spawn would even be useful to him. They can’t go out in the sun, they can’t enter doors without asking, and they can’t cross open water, and they can’t make more Bhaalspawn. DUrge should probably be melting while they’re walking off into the city alone.
Now that would be an ending. If the sun doesn't kill you, Wyll or Astarion himself hunts you down and ends you a couple days later.
