I brought this idea up when I was just fiddling with the idea, but I'm now using it in a real game with a real character (my own) and wanted to find the best way to do this (none of us are experienced DFRPG players).
So, I admit to ripping off the idea from the (amazing) audio series "Wormwood," where a main character is given a bizzare left hand in exchange for one he has lost by a mysterious (very demonic) benefactor.
1. This seems like a straight ahead Scion of Power w/ an Item of Power, since the benefactor can ask and expect assistance at any time, and ultimately has a VERY bad final request. Does this seem right? i.e., other's recognize him as a powerful tool (in both senses) so you've got the -1 refresh and the +1 bonus to dealing with supernaturals.
2. Assuming the hand is an Item of Power (+1 since it's not a sword but it's not a hidden secret lapel pin), it seems like the basics work:
a. it "is what it is" - it's hand like, and can do things a hand can do
b. it's indestructable
So, in terms of abilities granted, the general recommendation I received previously was to try to just make it full of skill stunts rather than trying to pick a bunch of powers and then gimp them with endless catches. So, here's what the hand can do (or I'd like for it to be able to do), and my stab at how to model it:
c. Perception Supernatural Awareness: the hand can sense magic, which it passes to its wielder like threads of various strengths that he/she cannot see but feel. e.g., a ward might feel like a strong metal mesh wall, while the tracks of a supernatural creature are a rope-like thread that can be followed. So, Awareness - able to sense magic and get a vague sense of "what it is" metaphorically. Cost: no idea.
d. Strength Crushing Grip: The hand itself is supernaturally strong, however since its just a hand, it's kind of limited. It can hang onto things indefinately, crush steel or someone's throat. However, it can't do much else because it's attached to a normal arm (no bonus to hand to hand damage, can't punch extra hard).
e. Buglary/Larceny - Open Any Door, Break Any Trap: The hand itself can supernaturally unlock nearly any mechanical or electronic lock or something that's simply "stuck" but would normally open. It can do the same thing to magical shields, wards, locks, pre-prepared spell traps/locations and the like. So, I was thinking its a stunt on burglary or larceny. However, I'd like ALL the points in the hand, not just the bonus, since without the hand the guy can't unlock anything without a key. The stunt would just be exteneded to work also on magical locks also. However, that's all it does - it doesn't make him better at any theivery stuff. Cost: no idea. Not sure how this would work mechanically either when not dealing with normal locks.
f. Dodge (or something like that): the hand will and can act independently to defend its wielder, moving at impossible speeds to catch a bullet, block an unseen blow, etc. Again, this skill and it's stunt bonus would be wholly in the hand. Also, while it's kind of powerful, it's limited - shoot him 3 or 4 times, the hand might at best get 1 or two bullets. Cost: no idea
g. Finally, the hand itself is incredibly creepy. It's sort of like a hand of glory made from a deamon's hand. I don't know if this needs to be modeled in any way, or it's built into the aspect.
Thanks in advance for your advice.