In the first instance Michael is trying to help Harry to do something he doesn't really think is possible..  So he suggests that if Harry gives up his power he can rid himself of the shadow...  It is like if he suggested if Harry stood on his head for three minutes each day he could cure his brain tumor..  When as far as Michael's knowledge of this kind of brain cancer is concerned, it is terminal, but there is an outside chance that it could work..   Technically a lie I suppose, but it is also a bit of a prayer and a hope that if Harry was willing to do that God would remove the shadow.

That's a bit more than technically a lie. I mean, what happens if Harry goes through with it, gives up his powers, and still has the shadow? Is he just going to be fine with it? Given that Harry uses his powers to protect people (and based on the books, it is entirely likely that Harry would have to deal with innocent people dying because he refuses to use his power to save them) I think if he realized that Michael had lied to him he would be far more likely to disregard everything that Michael says/has said...and given that Harry still has Lasciel's shadow...

Harry never handed him the coin and said, "here I reject this..."

No, but Harry gave the coin to Forthill, so I find it pretty unbelievable that Michael doesn't know.

In Proven Guilty, Michael says that Harry can get rid of Lasciel's shadow by giving up his power. Then, when Harry turns him down, Michael says that he has faith that Harry will find another way to be rid of Lasciel's shadow. However, in Small Favor Michael claims that there is no way to be rid of a shadow of the Fallen, and uses this as a justification for believing that Harry set up a little girl to be tortured.

Am I misremembering/misunderstanding, or is this an outright lie about a major issue by the series' resident paladin?

The crossroad demon doesn't "co-habit" inside the host, like the Denarians do.  They "demonically surveil" their contractees.  Mind you, they probably do it MUCH more effectively than the contractee believes, but pretend to do so much worse.  So the person who made the deal may THINK they have protected themselves (behind a threshhold, inside a Magic Circle, etc...) but they haven't.

In the case of your guy... the demon gave him back his magic -- the magic inside him IS the demon's link, so he can never ward it out, its already inside the wards!  That's not to say he can't EVER get away from the spying... but it's going to be REALLY tough!

Great idea!

Furthermore... the demon gave him back his magic.  It is, fundamentally, the demon's magic that he's wielding.

Not exactly. The specifics of this are that my character messed up Thorned Namshiel's plans and killed his host, and the next host he got had no magic potential. Namshiel, being really pissed about it, got ahold of one of my character's focus items and used it to essentially steal my character's magic for his host. My character then made a bargain with a crossroads demon to get her magic back--essentially a really powerful thaumaturgy ritual based on the fact that it's her magic, so she is a thaumaturgic link to it. So it really is her magic--that's why she needs sponsored magic in the first place.

For what the sponsored magic can do, I'm thinking something with conjuration/illusion, to represent the way the demon gives people what they want, only for it to turn out to be worthless/not worth what they paid for it, but I'm stuck on how it should manifest for combat. I don't want it to be pure illusions, and conjuration doesn't really fit for combat thematically (unless things happen like the conjured sword vanishing just in time not to block a strike--but if that happened, my character would learn to stop using it really quickly, which would rather defeat the purpose).

I'm mostly just importing crossroads demons into the Dresdenverse. I've only actually watched the first few episodes of Supernatural, but I've read summaries of some of the later ones and crossroads demons seem like something that would fit in well with the Dresdenverse cosmology.

In the Dresdenverse, most of what we know of demons&devils is about the Denarians.  In the earliest novels, Harry summons another (non-Denarian) demon in an effort to bargain for information; but that plot-arc gets dropped and doesn't reappear for the rest of the series (so far).

What we know Denarians can offer:
Hellfire (powerful, inherently destructive, unstable for ongoing use)
Demonform (Denarians mostly give their hosts access to a shapeshifted form; Anduriel seems instead to offer a "shadow" that can physically manifest in various ways, to do various things, without actually shapeshifting the form of its host-human, Nicodemus).
Information (secrets lost to the ages, or intentionally hidden, including spell & other magical knowledge)
That other (non-Denarian) demon only offered knowledge, I think...?  But we REALLY don't have much info on what overt powers can be granted by non-Denarians; fr all we know, Hellfire may ONLY be grant'able by Denarians!  OK, I grant that one's unlikely...

Point being, it's really pretty wide-open.

I think you just pick a theme, and the demon amps it up; Quintus Cassius had "snake magic" with or without Saluriel.  Dresden's own favored fire-magic and force-magic was MUCH more potent with Hellfire backing it.  Just remember that Sponsored Magic has to be something the Sponsor is willing to do.

I'm trying to make something that looks fairly different from the Denarians. My character has run across them before, and one of the crossroads demon's selling points was "oh, we're nothing like THOSE guys. They're just awful," so... (obviously, this isn't true, as my character will learn, but it ought to look true at first glance). 

Oh, and Crossroad Demons are happy to leverage the bargain in-hand to get MORE souls from others... so your character is likely to be urged/tempted to do things likely to put OTHERS' souls at risk...

That's the plan  .

"Who you are" is not just your personality. "Tall and lanky" is as much a part of Harry's character as his bluntness and lack of subtlety. Aspects are not limited to your personality, either.

Yes, but it's not one of his aspects, while Harry's "Not So Subtle, Still Quick to Anger" is, so we're back to the problem of how to mechanically represent being better at one thing within a skill than others.

I'm not a fan of the idea. I know some characters have skills to share and a shortage of Refresh, but this complicates character-building and seems likely to be problematic balance-wise.

Fair enough. How would you recommend dealing with skills where a character should be significantly better with one part of it than others, for example, if a character should be significantly better fighting with one type of weapon than others. Should that be handled with stunts, or is there some way to declare at character creation/milestones that you specialize in one thing and maybe get a bonus in that in exchange for a penalty for everything that isn't that? (ex. a character might get a +1 to weapons when using a katana, but a -1 to weapons when using something else)

The skills aren't really cross applicable, though. Think of it like guns and gunsmithing.

Evocation specializations mean you shoot the gun really well -- you can fire fast, hit your target, reload quickly, etc.

But they're not going to help you in building or repairing the gun. That's a different skill set.

The thing is, knowing how to shoot a gun really well actually would help you in building and repairing one compared to someone who's never picked up a gun before. You may not know everything about guns, but you know more than someone who's never used one.

Evocation can't do things like "burning out an infection." That's just not something the power is capable of, any more than you can build a gun by shooting bullets.

Of course not. That's why I want to use thaumaturgy. The example I'm thinking of is a scenario that happened in another book series that used elemental powers. One of the characters was poisoned with a hypodermic needle, and in order to get rid of the poison a water mage pushed all the poison in her blood to the place where the needle entered and a fire mage burned it away. I'm trying to find a way to to do that in the Dresden Files. While what happened is clearly a thaumaturgy spell, it seems like being able to use Luccio's hair-thin fire beam should translate to some sort of boost in using this kind of precision magic, even though the fire beam is evocation.

I think it'll almost always be easier and cleaner to spend the Refresh that would've gone into this hypothetical Power on normal Thaumaturgy refinements that match your Evocation specialty.

Probably, but it still bugs me.