It had occurred to me that one person that we heard much about is Morgana Le Fay.
Facts:1) She definitely existed in the in the Dresden universe at one point.
2) It's her athame that caused Lea's craziness.
Very likely true3) The "Le Fay" moniker is currently used as an honorific, to denote someone who travels a lot in faerie. Morgana was either "Le Fay" because she traveled a lot (like Maggie Sr.), or maybe the title was originally introduced in honor of Morgana. Either way, it's safe to assume she had her Ways.
4) Lea (and WoJ) say that the Athame is power because of who owned it. I think it is safe to assume Morgana was a
very powerful witch.
Speculation5) In the myths she is associated with Merlin, usually cast as his apprentice, usually as an antagonist. Given that her athame apparently passed infection along to Lea, it certainly sounds possible that she was infected at some point.
6) Given that Merlin seems to be a "good guy", it is possible that Morgana was always bad. Either that, or she had good intentions but was infected.
7) Given Merlin's mastery of more technologically advanced magic, I wonder if the same might be true for Morgana.
WoJ is that Merlin is "sorta alive". What about Morgana?
And finally the WAG: - Morgana was a great NN traveller. In her travels she is the first person to run across Nemesis. Perhaps she found him Outside, perhaps she ran across his prison somewhere. In any case, she is the first true infected person. Bad things start happening (maybe a mini apocalypse).
- Merlin (her mentor/lover/etc.) finally shuts down the bad guys by creating Demonreach, traps a lot of the Old Gods, and generally saves humanity. As an aside, he also creates the White Council (with it's anti-Outsider Laws).
- Ok, this sounds a little too much like "The Magician's Apprentice", but maybe Merlin deliberately traps both Morgana and himself inside Demonreach in the hope that someone will eventually be able to cure Nemesis corruption?
Didn't Nic Cage help with the Dresden Files TV show somehow? maybe he stole some ideas from Jim's secret notes?
- Nemesis isn't completely beaten (Maybe Merlin wasn't a Starborn), but he has been trapped/slowed down. Fast forward a couple of centuries, and a new wizard named "Le Fay" comes across Nemesis' prison and gets infected (or maybe manages to escape). Realizing the trouble she unleashed, she decides to create a weapon that will finally end the threat -- a "Starborn", a wizard that has special powers over Outsiders.