I'm going to post this here and hope that it gets buried before too many people see it.
This is how you can make any character the Bhaalspawn. Save before trying, as always.
1) Enable cheats
2) Pause the game and CTRL-K the PC and the NPC you want to make the Bhaalspawn.
3) Talk to the NPC and bring them back into the party before you talk to the PC to bring him back into the party.
CTRL-Y the NPC to kill him. You should get the movie of the hand disintegrating.
