I am most intrigued by the WoJ:
Q: Have we met the people who created the Hexenwolf belts yet?
A: That's another "I'm not gonna tell you" question. I will say, "kind of," "not really," and "yes." But we'll get into more of that during Cold Days as well.
I don't see the second part referencing anything but Nemesis, that would fill out either "kind of" or "not really" (probably the latter). Not really sure who could be the "yes" part.
FWIW, I personally never saw the Erlking as the origin of the wofl-belts. There is a certain "demonic" vibe going on there, something I don't get at all from the Erlking.
Other options include:
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Arianna: Strong Raith and RC connection, but maybe not powerful enough as a wizard. Also unsure of why she would take an interest in Chicago so far back, unless she was following up on Maggie's progeny.
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Tessa: Certainly has the magical capability and a "demonic" background, but the Raith connection is weaker.
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Personally, I'd vote for Mavra.
- Per the first two books, there's someone going around giving people magical help, and then in GP we see Mavra onscreen helping out Bianca and Kravos.
- Mavra has a grudge against the Whites, so belts ain't a bad thing.
- One of the PG theories is that Sandra Marlin (person who suggest that Molly use fear) is an agent for someone trying get Molly to use black magic (same as Victor Sells and Kravos). Molly met Sandra at a homeless shelter, giving us a definitive Mavra connection. Note that this might be the connection Jim was referencing when he mentioned that "someone figured it out".