It is heavily implied that the various god-like beings in the Dresden-verse, like the Faerie Queens or Odin or the Red King to a much lesser extent all have many masks or mantles that they can wear. Being that the Erlking is a force of nature not dissimilar to the Queens I am sure if something happened to him all that power would go somewhere.
I think Harry even quotes that about energy/matter often in magic. In fact, he said something about that in Changes about Lea's gifts before the battle. Again, more support for the idea of a "mantle" of the Erlking.
Also, if you didn't want to directly usurp the Erlking (as being that would be a big in-universe change) you could have someone pull off a version of Kemmler's Insta-God ritual or something like it. Absorb the Hunt itself.
Anyway, plenty of canonical support for your character idea. I think it sounds interesting - your character must have brass balls or else be completely nuttters if he wants to take on the Erlking
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