Firth 5, this is a possible spoiler! Don't read!
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Okay.
Can "wicked jar" be cast at "part" target so you have only part of a demon's might attached to the jar? Why would you want to do this? Because if you do then all of the demon's might can't be penetrated unless you have the jar, thus preserving the demon in case some pesky mage tries to kill it.
Interesting concept, and a classic one. To have some secret, well-hidden "vessel" that must be destroyed to allow ultimate destruction of the BBEG.
Don't know if that spell, specifically, could do it, but certainly something like.
I think it comes down to what the saga-specific paradigm is for demons, how ~you~ want them to be in your game. A "mind" doesn't have "part" (afaik), since its really not divisible. (I know they don't have "sizes"... hmmm...)
Since demons are not (! should not!) be overly defined, or limited by game-mechanics (there should always be an X factor, some wiggle room), I'd say go for it. Maybe that spell +2 magnitudes, 1 for Part, +1 more for "Complexity" (which seems to cover a world of sins.) 8)
Noble's Parma: Another, simpler option might be to have the demon act by proxy, investing all his might minus 5 points in possessing someone or something.
The possessed could then use the demon's power, be attacked through DEO, but the real demon would stay safe
Ohhh interesting. Wouldn't work for this story but its a very good idea.
Another simpler thing that just occurred to me is that some demons can split up into a group of small animals, like rats or crows. Each of the animals contains one point of the demon's might and if one of them is killed then the demon loses that might point. What would be cool, and suit my purpose, is if the demon split himself up and the mage took and imprisoned some of the animals.
If someone did this, they would have an arcane connection to the demon, wouldnt they?
Makes sense.
As for guidlines on the enchantment for the jar, I can't say. I would guess that it is a MuVi effect with a Rego requisite. Or, think "hermit crab". Do the MuVi thing to attach it to the bottle and then ReVi to create a mystical tunnel between the jar and the demon. Use Might points to substitute for vis.
Attaching the demon to the jar may be discussed in RoP:I but I don't have the book.
Maintaining the Path to the Secret Hiding Place (similar to the Opening the Mystic Tunnel spell in the core book)
Target: individual
Duration: Concentration (plus two uses per day to make it constant)
Range: Arcane Connection
Using the bottle would be an arcane connection in a very active way and would be classed as indefinite duration with attendent multipliers.
Destroying the bottle would damage the demon (loss of the Might points used in the "enchantment") but may not kill it, but would make it possible to dispell it since it can't retreat into the bottle any more.
Just a thought.
-K!
That would be D: sun, not concentration. Appart from that, it sounds OK to me. 
Anything that allows you to tell cool stories, is a good thing. If the rules get in the middle of a cool story, drop the rules, not the story. Been doing that for years and IMO is the way to go
Cheers,
Xavi
Thanks, Xavi! That's a good way to do a judgement call.
However, I do have to stick to the rules somewhat. This is my first shot at being Sg and I don't want anyone to go "wait a minute, you can't do that!" and blow a big plot hole into what I hope turns out to be a cool story.
All I need to accomplish is that the player can't destroy the demon until he finds the jar and breaks it.
Dunno if it exist in 5th edition, but in previous editions (3rd and 4th) there was a thing called an External Soul. Some creatures (usually trolls, dark mages and other bad guys of their kindergarten) had it. They could be mangled, but would not die until the item holding their spirit was destroyed.
Such abilities are popular in legends and fantasy literature, about the fiend that cannot be destroyed by brute force alone (last example being Harry Potter, IIRC). Give the demon an external soul virtue (the jar) and you solve your problem. How he acquired his external soul becomes only a background information item then, and no rules can contradict you at all.
Cheers,
Xavi
Ya, demons aren't using the Hermetic model of magic, so they can "cheat", so long as the cheating is consistent. (Or, at least, it "appears" consistent, as far as any investigating Player Characters are concerned.)
It's a lot like theater- what happens behind the curtain just doesn't matter. 
I want to make sure that none of my cheating breaks any rules. 
I loved the external soul and the trollsynir, I miss them
Well, that's fine, but also remember that demons don't follow the Hermetic model of magic, which means that they can do anything they pretty much want. Game balance is key, and it would be wise to put it into terms that are meaningful to Players/Magi (even if that "meaning" is a realization of "They can do WHAT?!")
For instance, Demons aren't ~necessarily limited by the Hermetic "Limits of Magic" - or they may ~appear~ not to be (Demons are great on deception, don'tcha know!) They can also, effectively, have made a "breakthrough" (one they are willing to teach/trade to magi??? For a price, natch...), so that's another way to push the envelope without breaking the rules.
If a demon was just a big wizard, it wouldn't be as much fun. 
ALWAYS be mindful of magical devices you have in the game...In this case, this item that holds the demons 'soul'/might...
If you break a limit, then its in the item...a magi can investigate this...and learn...
Obviously, this has the potential for many stories (warping, etc)..but remember that the Magi in question could come up with a HUGE windfall of knowledge...and if it will screw things up, they probably WILL.
That's what already happened. A maga cast this spell to protect the demon and she was appropriately warped. she was, however, killed for her crimes by the mages at her covenant.
Thats what my player gets for making a Tremere necromancer. Of course everything is going to go horribly wrong!
In my typical "I thought of that after I did it" way, I let my player play half way through the story and then got scared that I was doing it wrong. but it all sounds okay so far.
Yeah, thirty years and they still find ways to screw it up...
(sigh)
Thirty years of what and who finds ways to screw it up? 
I screw up plenty of things in way less time than that.
Gaming....My players always find a way to screw up the game if I don't pay any attention...no matter what game it is...
Belladona, if you check "The Infernal", you may see p106 an Incantation/Consumption effect that allow one to transfert his soul into an infernal object.
This might be useful to you
I have been combing through that book and I totally missed that! Thank you!
I was reading it on an airplane and the people next to me thought I was some kind of a satanist.
Evil Forever
And my pleasure