We do have Morgan right here, along with his presumably powerful coworkers. Now that they're out of that red court trap they may be able to toss around spirit messengers or something.
The only dealings Harry had with HWWB was setting him on fire.
If you're thinking of the deal he made for power around that same time, that was with Leansidhe.
Pretty sure we got most of them at least.
Edit:
Was thinking about this, and it sounds like this thing incorporates the souls of its victims to some extent.
Perhaps I'm reaching, but in DF canon killing a soul is really hard if it's possible at all, especially a mortal's soul. You can cripple, mutilate, and otherwise defile them, but destroying is another animal entirely.
So is there enough left that if we take the parasite-spell-demon thing to Sanctuary and hack it to pieces that the Wheel could put his victims back together again?
@DragonParadox does Molly know enough make a good guess about the answer?