OK, this might actually happen; my players have found a soul jar and are trying to capture the soul of a dragon lich in it.
It was casually floating around the dungeon, waiting to be resurrected by an out of town lich lord, when the players barged in. Needless to say, they found its skeleton, and they discovered its plan (Charm Person on cultists was really, really hilarious) so the cleric cast Speak with the Dead on it, and discovered how to trap it, and where the soul jar was.
They will probably succeed in this, and the soul jar will probably become a custom magic item. The thing is; what should it do? I have some ideas, but I’d love to hear your thoughts on this 🙂