This is from the TV series "Supernatural," correct?
Are you using DFRPG/DFA rules to play in that 'verse?
Or "importing" a few pieces of Supernatural into the Dresdenverse?
Or doing a full-on "everything included" blend? (fwiw, I believe Jim views the Dresdenverse this way - it has at least pieces of most myths, folktales and fantastical novels/movies/TV)
Or ... ?
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).
- Damage resistance, and incredible healing
- 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.
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...